This is my version of "getting started with Warlokk's BodyShapes." It was originally posted at InSimAdult (and I suspect lost in the transition), but I thought I'd put it here and post links to it.
First, look through Warlokk's site, specifically the BodyShape Catalog. Pick one or two.
Next decision: Do you want the XHi-Res meshes with toes and better curves to the thighs and butt, or Hi-Res meshes with Maxis-style blockfeet? The XHi-Res have higher poly counts, which may cause some systems to lag. If your computer is a little laggy already, go with the Hi-Res. The female nipples are also in a slightly different location on the XHi-Res mesh, so you might want to poke around and see what skintones are available for your chosen shape.
Third decision: Do you want anatomically correct (AC) or Barbie nudes? I believe the XHi-Res is only available as an AC mesh, so the two decisions are interrelated.
Now you need to find the nude meshes. In general, the AC nudes are located in Warlokk's Tower on InSimAdult, and you'll choose the correct files for XHi-Res or Hi-Res. The other nudes are best located using TMO's page,The Mystical One's BodyShapes by Size with Links. You must have the top and bottom separates for your chosen size.
I also recommend getting the full body nudes (including the underwear and swimsuit meshes if available) because you'll need them to convert your favorite skins to fit the newly installed BodyShapes. Basically, if you try to convert swimsuits or casual clothing using an XHi-Res nude mesh, you'll have very visible camel-toe, butt-crack, and pointy nipples. The swimsuit mesh will minimize this while the underwear mesh has an intermediate amount, to represent covered by lightweight tight fitting clothing.
The next step is the skintone. I recommend JWilson5's conversions, either in her section on ISA or under her user name on Insimenator. Again, you need to make sure you have the desired BodyShape and variant (AC/Barbie, XHi/Hi-Res). You can also make a favorite skintone "showerproof" using the tutorials below. Verify that the skintone lines up with details in the mesh, such as nipples, sexyfeet, penis, etc. using Chris Hatch's True Nude in BodyShop or True Nude in CAS.
The last step, where you need to exercise some restraint, is to download some clothes for your chosen BodyShape. The place to start is Warlokk's Tower on both Insimenator and ISA. You'll probably want to get all the conversions of Maxis meshes. These meshes will allow you to convert many outfits to the new size quickly and easily.
Warning! InSimAdult and Back Alley Sims are adult sites and require visitors to be over 18!
Helpful Links and Tutorials
Complete Nude Collection for Dummies by a Dummy, a beginners guide by AnalogKid
The Mystical One's BodyShapes by Size with Links
Gothplague's BodyShape Conversion Tutorial
Warlokk's BodyShape Guide
Warlokk's Tutorial: Converting standard outfits to BodyShape Meshes
How to link a specific mesh to a genetic skintone using only SimPE. If you only want to replace one of Warlokk's teen meshes with one of PierreNC's Princess meshes, this is probably what you want.
However, since the above tutorial modifies an existing skintone, you may want to copy your favorite skintones using Yakov's Pixel Perfect Skintones tutorial first or use the tutorial below.
How to link a genetic skintone to a specific not default nude mesh