Step 1. Measure your waist, and multiply that number by 1.5. This will be your skirt width measurement. Now measure from your waist to your knee. This will be your length measurement. Cut two rectangles of fabric using these measurements. Then cut two WAISTBAND rectangles using the same width measurement, and making it 3" in length, as shown.
Step 2. Sew the sides of the skirt up with right sides together, then sew the sides of the waistband up with right sides together.
Step 3. Fold the waistband in half so it's now 1 1/2 inches tall and has a folded edge on one side and two selvaged edges on the other side.
Step 4. Pin the waistband of the fabric around the top of the skirt. Make sure the folded side of the waistband is facing down toward the bottom of the skirt, as marked in this step. Sew around the entire waistband with a straight stitch and a 1/2" seam allowance, but leave 3-4 inches open so you have a spot to slide your elastic inside.
Step 5. Measure your natural waist and cut a piece of to this size. Attach a pin to one end and use it to feed the elastic through the waistband casing you just created. Make sure the other end of the elastic doesn't disappear into your casing -- secure it with a pin as it gets close. Once both ends of the elastic are sticking out of your casing and the elastic is completely fed through, sew the ends of the elastic together. I typically use a serged seam to securely sew them together, but you can use a straight stitch or a zigzag stitch.
Step 6. With the waistband still folded down, like before, sew up the little opening in the casing.
Step 7. Fold your waistband up and press in place. Hem to your desired length and you're done!