I found this on a blog years ago. I'm not sure what blog it was.
But, I have used this idea a lot on all types of pants and shorts.
It works great every time.
Step 1 (Not pictured)
Put pants on the person and find where you would like the seam to be.
Fold the pant leg up.
Step 2 (picture below)
Measure the length that you folded up on the pants and divide by 2.
Fold the pants at the new length and pin.
Step 3 (picture below)
Sew right up against the old seam. I sew it twice around just to be sure.
Step 4 (picture below)
Cut the pants about a 1/4 an inch from where you sewed against the old seam.
Step 5 (picture below)
Add Fray Check all around where you cut the pants.
That way you wont have strings hanging down when your wearing them.
Step 6 (picture below)
Fold the old hem back down and iron.
Step 7 (no pic sorry)
Wear your new pants!
No one can tell that you did the hem yourself!