You need to know Blender basics. I'm going to show you my way to achieve the goal and sorry for my English. I used my experience, reviews, discussions, YouTube, other resources and these guides:
Japanese site[hal51.click]
https://steamproxy.net/sharedfiles/filedetails/?id=1412599164&searchtext=mega+guide
https://steamproxy.net/sharedfiles/filedetails/?id=1405767408&searchtext=guide+weapon

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Originally posted by Authors of Mega Guide:

To install this fixed FBX plugin, do not unzip the file! In Blender > User Preferences > Add Ons there is a button near the bottom, "Install add on from external file." Click that and point at the .zip. Then still in Blender > User Preferences > Add Ons, make sure the FFXV FXB one is now enabled (ticked) and the standard FBX one is disabled (unticked). If you want to load any of the SE Samples, in order to use their armatures on your character mod, you need to change a setting during the FBX import or the bones will all be rotated by 90 degrees.

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀To save your presets for import and export:
⠀⠀⠀⠀⠀⠀⠀⠀⠀File -> Import/Export -> FBX -> choose right settings -> + -> enter a name.
⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀ Import Settings

⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀⠀Export Settings
Edit -> Preferences -> Add-ons -> Install -> select ZIP file -> Install Add-on -> select a checkbox.

• your Windows username shouldn't have spaces. Only English letters, according to randolf. If you have the spaces, change your user folder name in the registry, according to Eviloneiam. My suggestion is to think if this is safe for you. Their discussion;
  
• don't use the space in the name of your folder, according to BlackStarGabusGaben;
  
• don't use dots in the names in Material Properties or your game will crash. Use English, _ or space;
  
• don't use dots in your BLEND files;
  
• don't forget to create the new BLEND files from time to time. The previous ones are your backup. I used the numbers in BLEND files' names.

To sum up, better use only the English letters/numbers everywhere, including Vertex Groups, but you may leave _ in Vertex Groups.

1. Open Blender -> delete Camera and Light. To import the skeleton: File -> Import -> FBX (use your preset) -> hide the skeleton clicking an eye icon.

2. Import Noctis's model (XNALara add-on). File -> Import -> XNALara -> XNALara/XPS Model. The same for the model you found.

3. Import the model you found for the outfit (XNALara add-on).

4. Hide Noctis's armature. To see bones through a mesh: left click outfit's armature -> Object Data Properties -> Viewport Display -> Show -> In Front. I also prefer Display As -> Wire. Try to adjust the outfit to Noctis's proportions, to the bones. Change size pressing S then Pose Mode -> choose bones -> change their size, rotate R, choosing a direction X, Y, Z, maybe double R and so on. Shift + B and use the mouse to select the area you want to have the close up look or also scroll a mouse wheel.
Useful info: to resize the bone's mesh, without affecting the others, select the next bone after the one bone's mesh you want to resize -> Bone Properties -> Relations -> Inherit Scale -> None. Now select the previous bone where you wanted to resize the mesh -> press S.

5. Delete the head of your outfit in Edit Mode. Edit Mode -> A to select or deselect all, so... deselect -> C to select verticles (points, a singular form is vertex). Ctrl + + to select more around your selection, Ctrl + - to select less -> press X to delete -> Verticles. Try to have some mesh close to an end of Noctis's neck.
Useful info: if you don't have a numpad, emulate it Edit -> Preferences -> Input -> Keyboard -> Emulate Numpad.

6. Delete Noctis's model: in Scene Collection select a right Generic_Item -> right click it -> Delete Hierarchy.

7. Better create the new BLEND file from this one. Time to delete outfit's armature. To leave the mesh in its pose after deleting bones, do this to every mesh of your outfit: go to Modifier Properties -> the arrow under Add Modifier (if you have such a modifier)-> Apply. I didn't combine the meshes of the model in that step. Select outfit's armature -> holding Shift, select every outfit's mesh -> Ctrl + A -> All Transforms -> select outfit's armature -> press X -> Delete.

8. This will be tricky. Prepare yourself. To combine outfit's mesh and skeleton: unhide the skeleton for your outfit -> select every outfit's mesh (I select every mesh and don't use Scene Collection because had some problem, long to describe it, well, if you want... Ctrl + J to combine the meshes you need) -> select the skeleton -> Ctrl + P -> With Automatic Weights.
If you get an error Bone Heat Weighting:

• select every mesh -> Tab -> select all pressing A -> Mesh -> Merge -> By Distance. Do this to every mesh of your outfit: Modifier Properties -> the arrow under Add Modifier -> Apply (click "X" to delete a modifier if you can't apply). Select the skeleton -> Ctrl + P -> Armature Deform -> With Automatic Weights;
  
• if the problem still appears, copy everything into the new BLEND file: A -> Ctrl + C to copy -> Ctrl + V to paste into the new BLEND file. Do this to every mesh of your outfit: Modifier Properties -> the arrow under Add Modifier -> Apply. Select every mesh -> select skeleton -> Ctrl + P -> Armature Deform -> With Automatic Weights. If everything is OK, use this BLEND file;
  
• no? delete the skeleton, import it from the ZIP-file again;
  
• the last not the best but still an option: delete the material that is a cause of the problem in Material Properties. To understand which one, deleting any material, do the same from the first option;
  
• if that didn't work for you, then I'm out of ideas, but don't give up! Create the discussion or ask modders.

9. Now check if the mesh and bones move, and they must move after those manipulations. You may notice black spots on the outfit. This is a consequence of merge by distance. To get rid of them: select your mesh -> Object Data Properties -> Normals -> uncheck Auto Smooth.

10. Some explanation of Weight Paint. Combine all meshes Ctrl + J. Select your mesh -> Weight Paint Mode. Weight 0.000 is the coldest colour (dark-blue), it freezes your mesh and doesn't let it move. Weight 1.000 (red) is the hottest one, I imagine boiling where the mesh start to move at its maximum. Brush -> uncheck Front Faces - now some occluded vertices will receive weights too. Viewport Shading -> I prefer to switch between the 1st and 3rd spheres.

11. As you see Object Data Properties -> Vertex Groups - every group is connected with the bone. Better create the new BLEND file now. I suggest to delete some Vertex Groups we don't need, clicking -:

• from C_Neck1 to C_HeadEnd.001;
  
• from L_Soket to L_SoketEnd;
  
• from R_Socket to R_SocketEnd.

Useful info: in case you deleted the group you want to bring back, let's say C_Spine1: press + -> rename to C_Spine1 -> open your previous saved BLEND file -> from here copy your outfit to your active BLEND file (you can close the previous BLEND file now) -> G and then X to move your model to the right or left side -> select the mesh you want to transfer the group from -> select C_Spine1 -> hold Shift and select the mesh you want to apply the group to -> select your created C_Spine1. Weight Paint Mode -> View -> check Adjust Last Operation. Weights -> Transfer Weights -> Transfer Mesh Data that has appeared -> Data Type -> Vertex Group(s). Vertex Mapping -> Nearest Vertex -> check Auto Transform. Source Layers -> By Name. Destination Layers -> All Layers.

12. A process of weight painting. Select armature -> Pose Mode -> rotate, for example, L_UpperArm. The mesh is attached incorrectly but not always to L_UpperArm. In this line of the bones L_Thumb1 may cause this to happen or L_Thumb1, L_ThumbEnd, a pinky also likes to be a reason of all that.
By the way, Ctrl + Alt + B to remove that orange line. It appears after Ctrl + B that you don't need.
Bring back the default pose of the arm, selecting its bone in Pose Mode and press N. Transform -> Rotation -> W set to 1.000, XYZ to 0.000 -> N to close or you can select all bones -> Pose -> Clear Transform -> All. One reason of the wrong mesh is L_ThumbEnd. To erase those wrong verticles: enter Weight Paint Mode -> set a brush to 0.000 weight -> find L_ThumbEnd in Vertex Groups -> erase.
Sometimes it's not easy to do this, but keep erasing. Remember that some verticles might be almost dark-blue, but they are still slightly active, though your eyes can't see that. Thumb's attachments can be anywhere, so move the bones and check around the body. Once you find something moving in the wrong spot -> leave the pose as it is to see the wrong spot -> Bone Properties -> look at the name of the bone -> find the the same name in Vertex Groups in Weight Paint Mode or just scroll through all the groups by holding down up/down arrows on your keyboard, hoping to see not almost the dark-blue colour -> erase. The first sphere as you remember may help you to see those intersection points. So you know what to erase or paint.
It might happen that the intersection points are behind the other intersection points. Rotate camera, use Shift + B to close up, scroll the mouse wheel and do your best. You can also use Gradient and other tools like for the other wrong parts of fingers. In some cases you can unjoin meshes from each other: Edit Mode -> Material Properties -> select the mesh from a list -> Select -> P -> Selection -> Enter. To see this net in armature's pose: Modifier Properties -> select the icons (a triangle and a square) [a GIF below]. After all corrections are done, move the bones and see if you need to attach some another verticles to your current verticles -> weight paint. To have the same colour: select Sample Weight and select the colour of your current verticles' weight -> use Draw to paint. Move bones to see how they affect the mesh. Set all bones to their default pose: select all bones -> Pose -> Clear Transform -> All. Maybe you want to test the outfit in your game already? If you don't like the result, then you can continue to improve the outfit and check it in the game again.
Useful info:

• if your brush stopped painting, turn off all two squares near Weight Paint Mode;
  
• if you suddenly find yourself in some weird panel and ask yourself what the Dope Sheet is that? Or smth like that, just select Layout instead of Compositing or instead of any other panel there.

13. To see your result in the game, you need to apply textures, but the game doesn't let me add bump. They must be in PNG format. Use the online tools or programs you wish to convert if they have the different format. Open Mod Organizer -> Create New Mod -> Outfit (Solo Play) -> Next -> Noctis -> Next -> Create. Right click your Mod -> Open Asset Folder - move all your PNG textures here. Copy the path of this folder and save this path in any text editor. Go to Blender -> select Shader Editor and check Use Nodes. Delete all nodes pressing A -> X. Shift + A -> Texture -> Image Texture -> Open -> paste the path you copied -> select the texture -> Open Image. Shift + A -> Shader -> Principled BSDF -> сonnect the yellow dots with a line (Color with Base color). Shift + A -> Output -> Material Output -> сonnect the green dots with a line (BSDF with Surface). Select all nodes pressing A -> Ctrl + C to copy -> move to another mesh -> delete all its nodes and paste your created nods Ctrl + V. Open another texture here. Do the same to every mesh you have. Some meshes may have several materials. If you see a different material on your mesh, though you've done everything right, go to Material Properties and delete the ones you don't need. Now File -> Export -> FBX -> select your preset you created earlier -> paste that path of your asset folder to a file path directory -> Enter -> Export FBX. Before you check the outfit in the game, I want to show you Noesis Model Viewer. Open any folder of your Windows and paste the path. Copy the folder and paste it into your desktop. Open Noesis -> find this folder -> double click your FBX file. Now you can see how the textures will be displayed in the game. Hold right click to move camera up, down, left, right; hold Ctrl and hold right click to zoom in or out. I hope everything is Ok here. Moving to Mod Organizer. Double click your mod -> select your FBX format of Outfit Model ... -> Apply. Left click your mod -> Submit -> look at the error panel.
The error "[E]NotFoundWarning: data://" means, you have the wrong path to your texture in an Image Texture nod in Blender.
I don't remember other errors, but you can search for them in Steam guides. So there shouldn't be any errors. Open your browser -> your Steam Workshop -> click your mod -> Subscribe. Start the game -> apply your outfit there -> check it out. Problems:

• if it's invisible, you haven't applied texture(s) to some mesh(es);
  
• if there is some black areas on your outfit -> close the game -> unsubscribe and subscribe again;
  
• black areas haven't dissapeared? Go to Blender and try to export FBX again or better change anything in your weights or mesh a bit and export FBX, then start the game and see if the bug is gone.

14. If you need to continue working on the outfit... work on the outfit. :D Well, to check it out next time, you need to export your FBX file into the same Asset folder and Submit again, then unsubscribe and subscribe again.

15. Final adjustments. Double click your mod in Mod Organizer -> add Title. Better change the description which is on the left side in your workshop. It allows you to use more symbols. You can add the mod's name that will be displayed in the game -> Outfit Name. The same for the description. Other metadata... I just used the settings of one mod in WorkshopXD For the preview image I used 600x600 PNG. Use the same name - $preview like in the Asset Folder. Replace that PNG with your PNG.

That's all I know. I tried my best to convert my thoughts to the words. I hope I didn't forget anything. Maybe some of this information you'll find useful.
I want to thank the ones who created their guides and/or suggested something in the discussions, on other sites and