You need the weighed body blender file from here: http://www.imvu.com/groups/group/Blende ... 1,77010178
After you get that, you just model your new clothing item as a new object, unwrap and texture etc. Then you attach it to the skeleton by parenting and adding a armature modifier. You don't need to understand bones much in this case, but the relationship between bones, weights and the mesh.
Adding weights can be done easily by copying weights from the default body to your new object. You may need to repaint some weights if your mesh clips or pinches strangely with avatar animation since copy weights is not perfect. But basically each bone has a associated 'weight' which tells how strongly a part of the mesh should be influenced when that bone moves. The strength is indicated by weight paint colour. That same link also contains video tutorials that explain in better detail.
If you learn this process you can make most avatar items from clothing to hair. This was the tutorial I used to learn: https://www.youtube.com/watch?v=nqerNVlInNg
but there are new ones out there that might be better.
Other small tips that might help are:
- In the case of clothing you don't necessarily need to preserve the default body. If you are creating a tight fitting jacket for example, you can just delete the arms since you already have sleeves. This way you reduce file size and minimalise clipping.
-the cal3D exporter has some specific requirements. You need to have a UV map and a texture in each material slot otherwise it won't export correctly amongst other things. Bring up the Blender console to see any errors. If it did export but you can't see it, you may not have applied the transformations (with ctrl+A) so it is super tiny in imvu client. If it loads in client but explodes in a mass of vertices, perhaps you didn't apply a blender modifer like mirror or subdivision. Don't give up if problems happen! Just ask around
But if you follow steps exactly as shown in tutorials, you will be fine.