I am trying my hand at converting meshes from other games (Fable) by making a hat (a hood and mask combo actually). I have weighted it and edited its cut so that it appears in S4S, but instead of the texture it should have, it looks like it's made from human skin. I've been at this for hours and I want to learn how to fix this problem so I can start with the rest of the ensemble. I once commissioned an outfit for this particular character (not on this side), but I haven't heard back in almost a year.

It doesn't appear that you've imported your texture in S4S. Other than that, the UV Map should only be in a specific area depending on what category it belongs to. You can get a template of where to put the UV's here. http://sims4studio.com/thread/62/fix-uv-map-template

I've made some progress, it no longer looks like a meat mask, but the texture doesn't only get applied to the mask but the entire body. I suspect the UVs have something to do with it. Do I just import the uv guide image to blender? I'm not sure how to go about this.

Firstly, it is good to know that when dealing with CAS that there are two uv map to deal with. The uv_0 and uv_1. The uv_0 is the sims body, and the uv_1 handles all the objects that the sim wears from hair, rings, shoes, watches and so on. So the uv_1 is like a layer over the uv_0.

If you go to the link that was given to you above, you will find the template for the sims body that gives you a guide as to why e what is supposed to be. Meshing an object and applying a texture to it, is totally different from that if a CAS object. So the template comes in handy for CAS objects, to help you place the part where it to be.

In the page that the above link points to in Sims4studiio, the second image shows a purple colour at the top left of the template that refers to HAIR. What it means is that when you are creating a Sims hair, you have to map the uv of the hair to that location and at the same time apply the texture of the hair to that same location.

Also as a guide, if you are working on a CAS object, try downloading an object that is similar to that you are working on and look at the texture map to see where where that part is placed. Then you have to make yours too sit in the same location.

As a random observer, I just wanna say that meat mask is terrifying, full body mask is hands down wicked cool, and the accessory itself is great. Post pics when you get it done!