Zentangle is structured doodling. That's the best way to describe it I guess. It was made popular by a couple and many use it to de-stress and for meditation. It is a great way to unwind after a long day. I keep a notepad next to my bed. The repetitive doodling puts you in a lulled state and helps me sleep when sleep is elusive.
It's quite easy to get started. The original concept asks for you to doodle within a 9x9cm square template, but this is no strict rule. The next step is to divide this box into sections. This could be a simple circle at one corner of the margin, or a more complicated division.
Now comes the interesting part. Fill in each section with a doodle pattern. There are various patterns available on the
Internet if you get stuck. It can be as spaced or as intricate as you want. If something doesn't go exactly the way you want, do not erase or backtrack it. The whole idea is to let your hand and mind flow freely.
There are no mistakes - only happy accidents.
You can either keep it monochrome or add colour. This particular one I did on the computer. I still need to scan the ones I did by hand. Will update soon.
UPDATE: