Crescent Moon, is often one of the first Tangles people learn. It is the fist tangle Maria Thomas shows on the DVD, and many CZT teach it in the introduction to Zentangle®. In my opinion that is because Crescent Moon, is an embodiment of Zentangle.
The Steps as Showed below are as follows:
1: Along the border of the area you are working
in, draw a number of ½ circles, and fill them in.
2: Aura each partial circle.
3: Continue to add an aura to each circle.
4: Apply Shading.
Possible Tangleations include: Changing the fill on the moons (below I use lines in some,) change how the Auras connect/bounce off each other, shade of fill the auras. You could also add some circles floating in space, or change the moons to other shapes, but at that point we may be moving beyond Tangleations to a new tangle.
For more information on Crescent Moon, here is a link to the crescent moon page at Tangle Patterns.