Here’s documentation to get you used to ZBrush.
http://www.pixologic.com/zbrush/downloadcenter/documentation/
If you have any 3D experience you should be able to catch on quickly. If not maybe you should start researching how to create basic meshes using your 3D modeling software. I wouldn’t suggest trying to create thing’s from scratch using ONLY Zbrush as since you are making a MOD you need to follow strict model guidelines for them to animate/work in the Sims 2.
However if you do know about 3D here’s a good start on the workflow
-Import Template Sim 2 Model into 3D model creation application (Blender/Maya/3DsMax) [idk if your Milkshape allows you to create models]
-Modify existing Sim2 model or created one with identical UV map (not sure if Sims2 allows models made from scratch or just modified ones)[look up info on this step depending on your 3D software]
-Export that low-res mesh from Milkshape/Maya/etc and import it into ZBrush
-Proceed to sculpt/paint
-Export Texture and Normals maps from ZBrush
-Import both the modified low-poly mesh and its textures into the Sims2 (look up info on sims2 mods for info on this step)
Hope this helps some!
Goodluck!