Well, I’m not a good sewer, but I’m sure that with the tutorial for the non-rectangular magic bag even I can sew a bag.
I find the tutorial great! :)
By the way, the bag on the picture was sewn quick and dirty during the lunch break, just to see whether it works ;-)
Links:
Non-rectangular Magic Bag (Zaubertasche rund oder mit nicht-rechtwinkligen Ecken) (German)