How I did it:
1. Put the skirt on inside out and figure out how much you want to take it in on each side. This amount should be equal to keep everything centered.
2. Once you know how much you're taking it in, lay the skirt flat (inside out still) and put a pin at the top, marking that measurement on each side.
4. This step is optional: With a pencil or fabric pen, take a yard stick and and connect the pins. I do this to help me see the angle and to make sure it works on both sides. It also helps me keep the seam straight.
5. Sew both sides of your skirt and try it on.
6. When you know the skirt fits, you can trim the excess with pinking sheers or serge it.
7. Hem the skirt to your desired length, and you're done! Just remember to always hem last because taking it in will make it sit differently when you have it on, even if it is just a little bit.
I hope this will inspire you to give things that don't fit a chance at the thrift store!