Well, this was my attempt..
It turned out quite small. Mind you, my shirt was smaller so the width of the biggest piece was 17inches instead of 20.
I really only followed the first few steps. By then, the bag was already so thick that I didn't want to add the two small pieces inside! And I just sewed the straps on rather than doing it like a drawstring.
Still, I'm glad I know how to use a tshirt for a purse now! Especially using the hem as the straps. Thank you!
Now that I know how it works, I'm going to make one with some nice fabric.