How do you make/sculpt your own sim

[Haszashi]

This isn't just sim related but mostly Blender related as well, I've been trying to figure out how people make their own sims like do they use Blender and sculpt them from scratch with a reference or what? I can't seem to find any tutorials for this or how your sculpt is exported to sims when your finished.

[Oops19]

People usually do this in CAS.

If you really want to sculpt a new sim in Blender you will soon figure out that there is not only the normal UV map but also a uv_1 which must match the mesh and some details are known about it, others aren't and as such a completely new sim has issues. Also generated normal maps need conversion for TS4. It's really a pain to 'backport' sims from Blender to TS4.
One can of course sculpt only the upper body and keep all seams to make sure that it matches the lower body, arms and neck seams.
And one should make sure that the existing outfits (these are meshes which replace the current eg upper body mesh) still fit.
 

Replace a full-body outfit with a head is easier but obviously not the same.