This tutorial will allow you to take any clothing item and make it a layer-able clothing item.

The previewer is a funny little toy with bugs.... it doesn't like to see a "no mesh" situation, so when you deleted the mesh id, it put it back, because there has to be a mesh in the product.(even if it's a single polygon) So, what you need to do is the following:

1. Go to the geometry tab
2. On the body part section, click on the drop down box and select the appropriate mesh for where the product would be. For example, a shirt is usually on the torso area so you would put back the torso mesh "male01.anime01.nakedTorso.xmf" or "female01.anime01.nakedTorso.xmf"
3. Click apply and watch the shirt disappear to the default avatar.
4. Click Add under specified body parts, and type in the mesh ID you wish to use(please check the forums or other products for which ID you should use for this layer so that it doesn't cause conflicts.)
5. With the new mesh ID selected, click the drop down menu on the body part section, and re-select the product mesh for the shirt(or whatever it is)
6. Click apply again, and the product should re-appear, but this time it is on its new mesh layer ID
7. Click back on the original mesh ID, and click remove, and apply changes (it should properly delete this time)
8. Texture, save, submit, enjoy!

Viola! It is now a layered product!