This is, honestly, the easiest skirt you'll ever make.
You'll need a bit of fabric, selvedge to selvedge (those are the woven edges, not the cut edges.) I like to cruise the remnant bins at my fabric store- anything over a 1/4 yard works great as a skirt for my oldest daughter!
You'll also need either ribbon or a bit of elastic. The ribbon should be about 18 inches longer than the waist, elastic should be 2 or 3 inches shorter.
Hem or finish both long (cut) sides of the fabric. Fold the top edge over 1 inch and sew to make a casing. (On this skirt I used biase tape on the bottom edge, and serged the top.) If you're using elastic, fold the casing to the INSIDE of the skirt, if you're using ribbon fold it to the OUTSIDE.
Sew both short (selvedge) edges together, starting UNDERNEATH the casing.
Thread the elastic or ribbon through the casing- if it's ribbon, you're done. If it's elastic, sew the ends together, and hand-stitch the casing shut.