Avastar is an add-on for blender and it is NOT FREE. First of all: I'm not paid by Avastar, and I'm not part of the staff. But it is a very helpful tool which makes things easier. And I have learned a lot by the documentations and the videos made by Gaia Clary.
Attention please: Absolut beginners will possibly have difficulties!
Okay, let's start. This is what I use for this tutorial:
Versions used (on Windows 8.1):
MakeHuman 1.1 (nightly build, 30.06.2015) http://www.makehuman.org/content/download_nightlybuild.html
Blender 2.74
Avastar 1.5-8
Sculptris: http://sculptris.de.softonic.com/
Mesh templates for clothing by Damien Fate: download here: http://blog.damienfate.com/?p=956 or here: http://www.outworldz.com/secondlife/posts/mesh-templates/
MakeHuman → Avastar → Opensim Part 1
You remember the steps you have to follow before you export your MakeHuman character directly to OpenSim?
Okay. But now you might want to create or modify clothes or the avatar himself.
First import your MakeHuman-Avatar to blender.
Now
1. Select the armature
2. Press the Avastar-tab
3. Delete the dummie
4. Press: transfer joints
5. Convert to Avastar
The avatar will be rotated.
Now create an object (add mesh, subdivide, apply material and texture). Never forget to apply rotation, location and scale.
In the Avastar-tab change the armature from „classic“ to „fitted“. Select the armature and your object (shift), press „Bind to Armature“, wait a moment, done!
Select everything you want to export, the body, eyes, eyelashes, but not the armature. In the Avastar-Collada-tab (on the top) press the Collada-button. Everything else will be done for you by Avastar. Just name your new character, export and upload.