These were the first true skins with anatomical correctness that I am aware of made for the Sims 4. Simply extract the .zip file, and put the .package file into your Sims 4 mod folder. Any time they are naked the bits will show. These are textures only, they do not remove the censor mosaic. See below for a link to that mod.
Required / Recommended Mods:
If you want to see the details when they are bathing, you need a mod to remove the censor mosaic. There are several of them out there. I use moxiemason's Mod at MTS.
If you want the sim naked while they are not bathing, you need some invisible clothing. I use Snaitf's . You can use any iteration of CAS to put them in the invisible clothing (creating a sim, planning clothes using the dresser, etc.). You need to click the x on the category type for the invisible clothes to show up.
If you have none of the above you will NEVER SEE THESE TEXTURES!
For Comparison:
Standard Skin: Shiny nipples, genital are sort of a mess. A clumsy first attempt.
Mk2 Skin: More natural, imperfect nipple shape. Genital with prominent clitoris.
Mk2.5 Skin: Nipples cloned from Maxis Male skin. Genital with prominent clitoris.
Mk3 Skin: Nipples cloned from Maxis Male skin. Genitals are a simple cleft, clitoris and labia are hidden.
Only use one skin replacement. Do not try to use more than one, as load order will determine which you will see . . . I think.