The first step is to create a pattern, i chose to make a pattern of four triangles rotated around each other. Then I used a pathfinder option to create this.

Then take your pattern and create it into a pattern.

place your pattern on whatever shape you want it to be, and to center it, make sure to use the tilde key, the hey to the left of the number 1 on the keyboard, on both macs and pc's.

Center your pattern to your liking. The tilde key allows you to drag the pattern through the background, and not drag the entire patten itself.