First a video, drawn by one of my favourite doodlers, in which he also explains how he finds new patterns. His drawings are spectacular. You can link to his website from this video at YouTube.
I can't compete, so I won't try, not least because I don't have a bison skull handy!
Last night I damaged a large sheet of raw watercolour paper I was going to use for a large doodle with water media added. I didn't want to lose the whole sheet so I got my cutter out and made some 10x10cm bits with the intention of trying some tiny drawings for a change from the big ones. My cutter can make a wavy line edge so I used that and it's decorative on the originals but will not scan properly, so I have taken it out of these copies of the doodles. I cut an undamaged sheet of paper up, too, so I now have about 20 little supports to decorate, and I'm considering using one or two of the official strings you can find here. At that link you will also find all the officially sanctioned socalled "tangles", which are the patterns designed by the founders of the zentangle movement. But lots of others are included and you can find lots more e.g. here or here. I usually draw my own strings (curved or straight guidelines in pencil) and then as likely as not ignore them. You can see from the video that the artist is working without any guidelines, but he probably has a vision of what it might look like! It's advisable - if you are not this artist - to design your mandalas in detail before adding patterns.
The first of my tiny doodles got its colour later. The drawing isn't very good and started as a scribble to use up the paper. I needed time to mentally switch to the small format. By the time I started 'floral' I was starting to like the tiny size so I'm hoping for better results today!
Below are a few old sketchbook entries (about 13x18cm) which I have never published. They are much more skillfully drawn and probably took ages. The secret is to have really fine-pointed micron to work with. They are not edited for colour correction.