It takes about 15 minutes and is a challenge, trust me.
This is how it goes:
Do a simple drawing of subject matter
Carefully count how many strokes you will need to cover the canvas
Pre-mix enough color to cover entire area
Load your brush and go for it!
The bigger the area, the bigger the brush
Do not pick up the brush until you have covered the planned area
Keep count your strokes
Give yourself one "Mulligan".