Put your jeans on and measure the length you want your skirt to be. Place pins.
Fold your fabric and place the jeans on the fabric.
Draw around the jeans . Make sure you made it a little bit bigger. You can always make it smaller at the end. Cut it.
Zigzag, fold and sew.
Cut one of the two pieces in half.
Zigzag the sides.
Fold the zigzag seam and place the zipper. Make sure the zipper is shorter than the skirt. Zip, so the two half are attached.
Place the two parts on each other, good sides facing each other. Sew, try it on. Does it fit well? Zigzag. otherwise, make it smaller and Zigzag. You can't make it smaller after the scalloped edge.
Open the zipper and measure the whole length. decide how many scallops you want and make a template. I can't really explain how but there are many tutorials on this site.
Draw the scallops and cut the out.
If you have a little duck tail, turn the zipped skirt inside out and sew a little triangle (blue dotted line) Try it on and make bigger if necessary.