Step 5: Sew!
Remember not to sew entirely to all of the outside edges in case later you want to slide one shirt in-between two other shirts.
It helps to start from the outside edges and move towards the center. Any which way you do it, if you're making a sizable rug, sooner or later it will turn into a seemingly unmanageable mess of fabric.
Don't forget to sew down the neck holes on the shirts or people could trip and hurt themselves!