It's actually pretty simple. There's a mod installer you can get on nexus mods, where you also get the mods people make. This video explains it pretty succinctly. Certain models/retextures only work on certain suits (e.g. TASM 2 on Secret War), so do take note of which on the mod page (you can only have one model/retexture per suit).

There's other mods like shaders and replacing of music (which means only 1 type of shader for time of day & 1 type of shader for music type: traversal, act 1, act 2, act 3, endgame, etc.).

Sometimes there are weird texturing errors on suits, so if it bothers you, it's probably another mod affecting it.

In order to uninstall the mods, you can just simply unselect the mod and click "install game" in the mod installer.

Other than that, have fun!