https://theslyd.tumblr.com/post/146300547576/tutorial-how-to-bake-shadow-map-for-cas-items

NOTE: This tutorial is using a dress and an accessory coat as examples. The method applies to any CAS item that you want to bake its shadow, including necklaces, glasses, hats, rings, bracelets, shoes, clothes of course, etc.

The shadow map draws shadow on the underneath meshes (which could be either a sim body mesh or another CAS item) covered by your item. It adds depth and makes your item a little more realistic.

Instead of hand-painting it, this tutorial will show you how to use the blender baking to create a ‘correct’ shadow map with just a few steps.

Please read my tutorial of How to bake shadow for clothing to make sure you understand the basics of baking. This tutorial will not cover some similar steps again.

NOTE:

• This method only works on the mesh where it’s able to tell the sim body from the clothes or accessory you’re making.

• It does NOT work if your item is painted on the sim body mesh.

Pt. 1 Your mesh includes part of the sim body mesh, e.g. tops, pants, dresses.

1. Open your mesh with blender, go into Edit Mode and select the faces which belong to the sim body.

2. Press P key to separate the selected faces into a new mesh.

Step 1&2 isn’t a must. But it makes the baked result easier to edit later.

3. Select the sim body mesh you just separated.

4. Open the World panel with a globe icon.

5. Check Ambient Occlusion and Environment Lighting.