1/2 yard fabric (For adult size, use 1 yard fabric).
1 spool elastic thread.
1. French Seam - back: Begin with fabric folded in half, with wrong-side edges together. Sew 1/4 inch from outer edge (if there is fabric information printed along these edges, or a strip of plain fabric, you'll want to sew 1/4 inch from the inside edge of that and trim the excess off, so that you will not see it when the seam is finished).
2. Turn the fabric inside out and press seam. Sew 1/2 inch from your pressed edge to create a french seam, hiding the unfinished edge inside.
2. Fold and press both hem and waistband (top and bottom) edges under 1/4 inch.
3. Fold and press waistband edge another 1/4 inch.
4. Fold and press hem another 1-3 inches (or 6 inches if you like, depending on desired length of the skirt).
5. With presser foot against the inside fold, stitch around hem.
6. Fill an empty bobbin with elastic thread. I use my machine threader just as I would for regular thread. You will have elastic thread on the bottom and standard thread through the needle on top. Set your machine to the longest stitch it allows (a "4" on my machine) or a zigzag stitch (both have worked well for me -- it just depends on what kind of stitch you want showing on the outside). With the fabric right side up, stitch just as you would for a normal stitch, with the needle in the center position, beginning with the presser foot on the outer edge of the fabric. This will both begin your rounds of elastic stitching and stitch down your waistband seam (folded in steps 2 and 3).
7. When you reach the end of your first round of elastic stitching DO NOT STOP. Simply stitch over the first few stitches of the first row, and then sew at a slight downward angle for 1/2 inch until your presser foot can use the round you just stitched as a guide. Continue going around in circles 6-10 times, shifting downward in a smooth drop at each circle-end. When you are finished with your last round, close your final circle by stitching firmly over the beginning of your last round to make sure the elastic thread won't unravel. Don't worry if the waistband seems very loose at this point -- it usually does.
9. Wash and dry your skirt on low (be careful drying elastic, as it can harden -- don't you hate fried swimwear?) Your waistband will be tight and firm now. After the initial washing, I usually hang dry any clothing with elastic (for the aforementioned reason, and also because I just like to hang our clothes), but you are more than welcome to dry the skirt howsoever you normally dry clothing with elastic.
Then throw it on and enjoy!