Hey!! I've been searching on this forum and couldn't find a tutorial or a start-to-finish step by step post describing how to make such outfits! I'm really sorry if there is and I missed it, just reply the link to me! Anyway, I'm trying to make a full body costume like the mario and bonehilda costumes out there existing for sims 4. The model I'm using has sort of a messed up UV map in blender(it overlaps on itself, not sure if this is gonna be a problem for sims 4 or not) along with no UV map for sims 4 and multiple texture files so I'm not even entirely sure this model could be used but I may be over complicating things completely. I've made standard recolors, skins, mesh edits, and accessories but I dont think the process is quite the same for those as it is full body outfits, I read on a post to use CAS tools for bone auto assignment but I couldn't get it working for me. If anyone needs the model file to look at or has any solutions or guides please let me know! I been trying to figure this out on my own and through similar guides for about three days and no luck due to my inexperience with cc making and blender. However I do know how to export meshes in S4S, append, and vertex paint if that makes a difference. Sorry if this seems confusing or wayyyy too complicated! if it isnt obvious I'm pretty new to the process despite playing sims 4 since it was very released lol.

I don't know of any tutorials specifically for full body outfits. In general it's the same process as any other clothing, except you clone a game full body outfit to start with, and use a full body mesh that's close to the same shape to transfer weights. You might have to merge some mesh groups to get a complete mesh including head and feet to transfer weights from.

You should give it a try and come back with specific questions and problems.

After I do this, should I vertex paint it then make the UV_1? And is the UV required if I don't want it to react to sliders or will it mess the mesh up? I actually have tried this and then attempted to import but nothing happened in S4S. Definitely I am missing something!

OK idk what I was thinking when I wrote that, however I did do it again and attempt to import into S4S again and nothing happened, no error or anything it loaded but nothing showed up in the model thing except the old mesh that had been there.

Just as a guess, maybe the cut numbers of your mesh aren't correct. Sorry, but I don't use S4S and can't help you with it more than that. You should ask on their forums.