You've got an old short too big for you?? You wonder why you bought it cause it's not your seize? Belts are useless? That's for you! Forgive me if there are langague mistakes, I'm French ;)
Sew elastic tape inside the leg of the short with the zig-zag point. Pull on the elastic tape while you're sewing.
Do the same for the other leg. Now let's do it on the waist. Stop before the button and the fly if you got one.
Now you're done! You've got a lovely short in your seize!
That's the back! I hope you enjoyed ;)