Chuck Moore has created a nice manual.
If you want to write a simple app to get a feel for it, first open the editor (type 'e' 'space') find and empty block. Then type some thing like:
ok show black screen green 120 75 at 200 125 box keyboard ;
Where 'ok' is red, the rest are green. Load the block ('xx load '), type 'ok', and you should have a green box on the screen. colorForth comes with other examples, but these might not fit on your screen.