I found this tutorial on PearlSims while looking for something else!
Pick body type you like. I picked 34Denh36 for my Sim, Angelita, so we'll use her for an example. It could be Rio, or Pinup, or RenGal, or 36DDnat40, or any of Warlokk's gazillion other meshes.
The mesh I need is in Warlokk's huge Nude Meshes archive. If it's one of the cup size/hip size meshes, you'll have to extract the whole ugly thing and just install the pieces you need. The named meshes are much more friendly -- You can just unzip and assume all the decompressed files get installed.
Poke around and see if you can find a skin tone you like for that body type. Jwilson5 has a ton of beautifully converted skintones by various makers.
I'm actually using the textures from a different artist with the mesh setup for 34Denh36 with Marvine's Slim Body Builder meshes for the guys. This means I need the Slim BB stuff as well as the 34Denh36 stuff. It works just the same, except that you'll need the corresponding files for that body type in addition to everything else listed in step 4.
Also get Warlokk or TheMysticalOne's Maxis clothing conversions in the appropriate sizes from the main Insimenator site. (This was one thing I couldn't figure out!)
Installing all the pieces: a) nude top mesh, b) nude top recolor, c) nude bottom mesh, d) nude bottom recolor, e) full body nude mesh, f) full body nude recolor, d) the nude skintone, e) your choice of clothing from step 3.
Make a Sim: Start up the game or BodyShop. Make your Sim as normal except for the following:
When choosing the skintone, you must make sure you've got your custom sized skintone, not one of the Maxis ones. In BodyShop, all the skintones are displayed side by side. In CAS, you have to click the asterisk (*) to see the custom skintones.
When dressing your Sim, first choose either the nude full body or the nude top and bottom to see how your Sim will look in the nude. Make sure the nipples are where you expect them to be (over the nipples on the mesh, not wandering off to the side) and the other details are acceptable.
If you've got multiple "sizes" installed, make sure you've got the one you intended. Warlokk requested that artists include the size in the icon for the skintone and in the tooltip text; most people seem to abide by that.
Now dress your Sim in the desired clothing. Again, Warlokk asked that people make their stuff clearly identifiable, so it should be fairly easy.
Save your work.
If you were working in BodyShop (better controls for fine details), you'll have to import your Sim into the CAS screen in the game. You use the icon that looks like 6 stylized Sims to do this.
To use Warlokk's meshes as nude defaults: (so your Maxis-sized Sims will have nipple and labia outlines, and look very realistic with custom skintones when they get naked), copy MESH_AFtopNaked_Hi_default.package and MESH_afBottomNaked_HiDetail_Default.package into your download folder.
MESH_afBody_34Bd-34_Naked_Hi.package and Warlokk_afBody_34Bd-34_NakedHi.package are also good things to put in there so you can check how your nude skins look. Many custom tones don't have their nipples in quite the right place to work nicely with Warlokk's work. It's easy to fix, but tedious!
The exact filenames will be different if you're using XHiRes or sexyfeet. I'll try to sort out the various files and sources soon.
Chris Hatch's True Nude hacks help you verify that you have all the meshes for your skintone, and that the AC skintone details line up correctly with the mesh.
TheTigerKing83 suggested I host his BodyShape Organization Kit. Finally, clothing for each BodyShapes will have its own icon and be grouped together in Body Shop and the game! Especially handy since Maxis helpfully broke the Tooltip Text!