The biggest problem I find with Zentangle is deciding what I am going to do next. Because I have such a bad memory (menopause brain, lol) I have created a book that has patterns in it so that when I am working on a project I can go through the book and pick a pattern to use. It is such an easy way of working.
I use an A5 Visual Art Diary and on each page I draw a 9cm square and do the main pattern in that. I then draw smaller squares and show how to do that pattern.
Then when I am working on a project it is easy to decide what to use and how to do it. There are over a hundred Zentangle(reg'd) patterns that can be used and you never know what you may create yourself, so having a book with them in is handy.
You don't have to use an A5 diary that is just what I found useful. Other people are using 9cm square paper tiles or even small tags held together on a ring. Try to find a novel idea that suits you and the way you like to work. Let me know what you come up with. It can be fun.