Note: Be sure to melt the ends of your cord so it doesn't fray!
First, you are gonna make a noose on each end of the cord. To do this, make an "S" shape at the end of your cord. Take the bottom of the "S" and loop it around the rest of the "S" two or three times. Now take the working end and put it through the existing loop. Now pull each side of the cord to tighten.
Repeat this on the other end.
If tied correctly, you should be able to adjust the noose from super small to extra large.