NOTE: When working on large canvases, it's easier to experiment with composition and colors if you begin with sketching out your designs on either a small scrap paper or a drawing tablet. Test out different colors, plant types, and shapes until you're satisfied with your design.
Lightly sketch your design onto the canvas using a pencil.
Mix your colors and fill in your pencil lines on the canvas. This may take two coats.
Use either black marker or black paint to fill in the plant lines over the colored shapes.
Touch up your design as needed; erase any leftover pencil lines.