To begin you can go to https://www.moddb.com/games/star-wars-battlefront-ii/mods , a trustable modding website containing every mod I installed. From here you can pretty much search out the ones you want and do the rest yourself but I will still showcase two must have mods and how to install them.
The mods I'm talking about are part of the BF2 Remastered Project by Harrisonfog, Javitolo98, Calrissian97 and MasterChief. The project includes improved graphics, remastered maps and a skin changer.
You can find these mods here:

https://www.moddb.com/mods/improved-sides-mod/downloads/improved-sides-mod-2-skin-changer-beta (skin changer (in beta))


https://www.moddb.com/mods/hd-graphics-mod/downloads/battlefront-2-remaster-installer-10 (graphics- and map remaster)

Note: Technically the second mod already contains the skin changer but it doesnt seem to be the newest version probably because the newest version is still in beta.

Harrisonfog etc. are still working on this amazing project (last update September 2020) so you should frequently check out the latest updates on his website https://harrisonfog.wixsite.com/swbf2modding.

Now to install the mod you obviously have to first click download, a window will pop up saying
"Click to download Improved_Sides_Mod_2.2.zip if it doesn't start automatically or try another mirror." this is quite self explanatory, then after about 5 seconds the download will start.
Harrisonfog has a tutorial on how to install, for example, the skin changer mod here: https://www.youtube.com/watch?v=FYGyPPE81_s&t=75s
and explains it on the download pages of both mods.
Basically all you have to do is unzip some folders and replace the BF2 folders that the mod contains e.g. if the mod has a folder called "fpm" and that same folder is in the BF2 folder then just replace that one (Attention: always make sure to make a backup for these folders). The directory for BF2 is C:\Program Files\LucasArts\Star Wars Battlefront II\GameData\data\_lvl_pc or C:\Program Files\Steam\steamapps\common\Star Wars Battlefront II\GameData\data\_lvl_pc .

Note 1: Some mod folders go directly into that folder and dont belong into any sub folder, for example "custom_gc1" and "user_script9" in the skin changer mod but as always this can all be found on the download pages listed above.

Note 2: The graphic- and map remaster mod now does this whole procces automatically

Now you should be done. This tutorial probably doesnt apply to every mod but this is a simple guide and most mods have a tutorial on how to install them on their own download page.

Hope this helped.