stormloader is the easy to use installer for your modding needs.
it contains a repository for mods that creators can upload and users can download, using the repository is very easy, simply find the mod click get and you are done. stormloader will automatically download and enable the mod.
if you find a mod that isnt on the repository you can simply click the "add mod form file" button, these mods will be a .SLP file. (SLP is a renamed .zip that has the files inside setup in a way hat lets stormloader automatically open and install them with no fuss.) the normal place to get mods that are not on the repo is the SMF discord in the quality controlled mods chat or the non quality controlled mods chat. the mods that passed qc will normally be higher quality and reasonably balanced with few exceptions

mods can be enabled and disabled at will by clicking the check or X to the right of the mod name or they can be removed from the list all together with the trash can button. on the left of the name will be a check or X indicating weather the mod is enabled or disabled, no need to dive into game files.

stormloader can be obtained by visiting the github and clicking the zip file at the bottom as seen in this picture.
it will start downloading, when complete it is important that you create a folder to put it and your mods in to keep things tidy and running smooth. once in a folder you can extract it. ( i use winRAR so in my case i would right click it and use the extract to stormloader vx.xx so it has its own folder within the mod folder so none of its internal stuff gets accidentally messed with.) once extracted you will open stormloader using the exe found in the folder, once opened you may be presented with something like this. (but without the top path filled in) also make sure to give it any permission it asks for.
now your path may be the same as mine but you can make sure by following that path, if its not the same then you can right click stormworks in steam and select properties and select local files and browse local files, this will show you where it is. once the top field is filled in you can just leave the bottom alone as stormloader defaults to a file within itself.
assuming everything goes right you should have a functioning modloader.

once you enable the mod in stormloader make sure there is a green CHECK to the left of its name, if NOT then click the green check on the right. once thats done you can open stormworks the normal way or you can click launch in stormloader. once the game is open go into a workbench, select the parts inventory and look for or type the name of the part to make sure its there. (preferably both) this is a example for the radial engines added by ETPEM.the next step is to spawn them in like any other part, and enjoy.

how do mods work with multiplayer?
well actually very good, they do not make the game slower or any less stable then it already is.
now there are a few things to know though, if the server has a mod the client does not then that client wont see the modded parts, however if the server does not have a mod but a client DOES then they wont be able to use them as they will be removed from the creation on spawn and the part will be grayed out in the inventory.

will mods break my game/creations?
nope, mods will not harm anything. by their nature they are very safe and stable.
on rare edge cases a part can crash the game when you place it but this is very rare since modders test their mods before posting and some even get a peer review by the qc guys on smf so no need to worry. keep in mind though if a creation has a part that you removed the mod for then you cant use that creation until the mod is reinstalled. (well you COULD but the parts will be missing) once reinstalled the creation will again be operable.

stormloader: https://github.com/Lewinator56/StormLoader/releases
GitHub

SMF discord (for help and mods that are not on the repo yet): https://discord.gg/9HS7cb6

note: myself, SMF, stormloader nor the mods are affiliated with the stormworks developers, we are just members of the community who like the game and want to improve it in any way we can.
please mod responsibly and enjoy what we can make together.
if you wish to make your own mods the creator of stormloader has a helpful video on the process.
https://www.youtube.com/watch?v=k4eOPBk_LjE&list=PLj-cCszUm4U5yFVT5XsWNReIiskmxIYRk