There is no proper user manual for WorldPainter (yet). I tried to make it as self-explanatory as possible, and you should be able to figure most of it out by trial and error. Click every button and see what happens. Don't forget to try every mouse button, using the scroll wheel, and doing these things while pressing Ctrl. Also don't forget to look through all the menus.
Having said that, there is some documentation:
How to use the Populate layer
How to edit existing maps
How to use the Biomes Viewer