I use it somewhat too. Here is how I do it.

If polygons:
UV-map it.
Assign a file texture in some slot (color, probably).
In rendering menuset, texture -> 3d paint tool options.
Select what attribute to paint in file texture. Set up image options and save it.
Start painting. Save when done. The files can be found in projectdir/3DPaintTextures/filename/ .


Hope it helps.