This example sketch of a tree trunk shows how simple this technique is.
1. The ink outline forms the shape
2. I wetted the trunk with clear water
3. Then dropped in random (and I truly mean random) colours on the right hand side of the drawing
Yellow Ochre - Burnt Sienna - *Sap Green - Burnt Umber
*the Sap Green was the only one that I purposely put to the left...don't overdo even though it is tempting!
THEN when the colours are in, I 'carefully persuaded' the paint to drift to the left, but not too much. It's good to leave some gaps of paper showing.
Leave it to completely dry. The magic happened all by itself...in fact the painting PAINTED ITSELF.
Go on, give it a try.