For the first two Zentangles I did, I doodled a random shape, and then divided it up into sections and made patterns in each section.
After that, I traced a CD and divided the circle up. I'm not sure why I never thought to just fill the whole page. You could also leave a space for writing in. I did most of these while we visited our son and daughter-in-law. I'm SO glad we traveled when we did - our whether was perfect. Tonight, on the other hand, it is sleeting!
Anyway, these were fun to do, and really not as hard as they look. You just do one section at a time, and there isn't any right or wrong. You just put patterns into the sections.