Try getting to know your brushes better. Take each one out for a spin but not before you check out their shape. Study the brush, analyze it's edges and then pick it up. Dip your brush into any green paint and make some marks with different parts of the brush, the body, the tip, the corner if it has one. Then hold the brush in different positions with your hand, vertical to the paper, horizntal if you can, grip it way back on the handle and close to the brush itself.
Experiment with pressure on the brush. Press it so lightly its like a flutter of butterfly wings. Try pressing the body of the brush down to make the fattest part of your leaf, then use the lightest pressure and the pointiest part of the brush for the tip of the leaf.
Please don't even think about mixing the right color green paint, that's a whole other Tuesday lesson.