If you know how to crochet this should be pretty easy for you. It's basically having to do what a hook does without one. Now make a slip knot near one end. Now take the longer end and push a loop through the slip knot(photo). Pull the loop through. Pull tight. Keep looping the long end through the last loop about ten times.
Step 2: Finish
Now, with your last loop, pull up the part of the string that is in the middle and raised a bit(photo). Pull a loop through both of the loops. Now with the loop that you have, pull another one through that one. Repeat this process until you reach the end. Keep on doing this until you get the desired size.