If you get custom underwear instead of the nude top/bottom: Create a new folder in your Mods folder, named 'aaaPriority' or anything that comes before any other folder or mod in the alphabet, and put the NudeinCAS packages in that folder. There are separate packages for male and female. Works on: Teen, Young Adult, Adult, Elder.
Nude can be saved as an outfit in any category. With this mod, when you click the little 'X' for tops, bottoms, or outfits in CAS to remove clothing, your sim will change to nude instead of underwear. This mod is now obsolete! Please use the Sims 4 Anatomy mod, also available in this blog if your game his the June 2016 patch installed.