This lovely, short video shows one way to draw a mandala using a series of simple shapes. I find the tone is very akin to that of Zentangle: go with the flow, don't agonize about it, there are no mistakes, see what happens.
I tried this using a standard 9 cm. (3.5") Zentangle tile and added some shading. Of course, you could add color. Or do it in colored ink. Or not bother with the shading (although it adds a lot!). Or... all kinds of things!