Hi Alan,
Doing this from memory, the quick steps are:
1. decide which layer or precomp you wish to make a layermap. you can hide the layer in the timeline.
2. In the effects control panel, look for the layermap twirlie. choose the AE layer you want and remember the layermap number to which you’ve assigned it.
Now, back to the material editor. Create a new material or modify an existing material.
Look next to the top thumbnail in the editor. You’ll see a little check box and a pop-up box. In the pop-up box, choose the layermap number you selected earlier.
Now apply that material to the object of your choice.
Does this help?
Jon