The best thing I could recommend would be to watch Bob as he paints them. There is a particular consistency that you have to achieve with the paint in order for the bushes to appear the way you would want. I struggled with these when I was first painting 30 years ago. The trick is to mix a little liquid white along with your yellow, getting it to a thin, cool whip consistency. Load the brush very full of that paint. Use that rounded corner of your brush, and press gently, bending the bristles upward slightly, right over your dark paint. Form ring-like shapes and leave the dark to separate layers. I would specifically recommend watching Bob's 3-hour workshop video. He goes into great detail about how to paint bushes, what to do, and what not to do. I hope this helps!
Great info Parksd, wished that video was available on YouTube but failed to find it there. The one hour video is there though.