Find an old pair of boxers (ask your brothers/dad/boyfriend/buy some) and wash it. They should be big & loose on you.
Decide on the length of your skirt. Pin the shaded parts to the inside. Cut the middle part where the legs/crotch meet.
Baste it. Fit it. Remove pins.
Hem it twice. Cut the extra fabric after the 2nd hem. And you're done! :)