The top must first be washed to remove any primer (which may prevent the paint from setting properly), then ironed.
Start by painting the branches in one color on one side of the top: directly with the nozzle, pressing down regularly. Diam's bottles are rather supple, which is nice when the work takes a bit longer...
Each time you're about to draw with the nozzle, start on a sheet of paper, tipping the bottle vertically to avoid air bubbles. Otherwise, when you press down on the bottle, you run the risk of an air bubble escaping, and with it fine droplets that could stain your work.