My granny taught me and we learned how to work a sewing machine in school.
Buy two squares of material that are the same size.
Lay one on top of the other with the front facing in the way,
Sew round three sides before turning inside out and voila! You have a bag!
My granny can make drawstring bags but I can't. I just put a little handle on mine by sewing on a strip of the same material.
Hope this helps!