When I had all the sections decorated I outlined each section in black.
The last step is the scary part. I always think maybe I shouldn't do it because as you can see the painting above looks pretty good. The only problem is that it looks flat. To give it a 3-D look I paint in the ribs with black then dry brush a white highlight in the center of each rib. I also add some dark shadows in black where needed. It covers some of the detail work but gives the saguaro the dimension it needs. Here is the finished painting.