First make a loop in the tail of your fall with the loose end on the top. Put your cracker through the circle and put the cracker loop around the end of the fall. Hold the end of the cracker and the tip of the fall with one hand. Hold the other part of the fall with your other hand and gently pull tight. When you get it in the right location, close to the end, work it as tight as you can. If you have a leather fall, you want the end result to look like a strait fall with a cracker tied around it. If you have a nylon fall, you want it to look like the cracker is strait and the fall is tied around it. You use the same set up for both. The difference is all in the way you pull it tight.
This is easier if you see it, so here are a couple pictures.