My first tip is for all of you with skinny little girls. Thank heavens for adjustable waist pants but sometimes the elastic waist pants won't even stay up on my skinny little girls (they take after their dad) since I really hate to just safety pin them in the back I often run new elastic in these pants.
First, measure how much elastic you will need.
Pin a safety pin the the end.
These particular pants just happen to have a fake drawstring with grommet holes that I can run the elastic through but if your pants don't have that you can just open a seam inside the pants and then sew it up when you are done.
Make sure to secure the other end of the elastic so you don't pull it through. Once you have threaded the elastic through pull it through the same hole you started at.