Step 6: Thread the drawstring
If you are using a string that is a bit more difficult to work with, you can either tape the end to make a makeshift aglet or tape the end to a stiff wire, skewer, or other device that can be pushed through the drawstring hole.
Once the drawstring is through, you are all done! How far off is the 5-minute estimate? :)
* One final note: It later occurred to me that you can make a much larger bag by using the body of the t-shirt. I couldn't do this because my previous project had left me with only sleeves, but it would allow you to use up an entire t-shirt to make two small bags and a big one. You would just need to open up two sets of holes for each of two drawstrings.