I use a serger, but you can just as easily use your regular sewing machine. Use the stretch stitch function, or a zig zag stitch.
First you need to gather your materials:
A sweatshirt-weight jersey kind of fabric
A normal weight t-shirt jersey fabric.
Optional: Thick elastic.
Step one is to cut out your pattern pieces! You will need to take your measurements and adjust accordingly. The top of the short (which is the top edges on both pattern pieces) should be 1/4 of your waist size, plus a half inch on each side for seam allowances. Note that the front has a corner seperating the bottom hem and the side seam, and that the back is a smooth curve for the side seam/bottom hem. This will come into play when you sew on your trim.
Also cut your trim out of your normal weight fabric.. You dont have to cut it on the bias, just so the stretch goes perpendicular to the long part of the piece. It's gonna be folded in half, as will the waistband. I like to cut the trim at LEAST 40 inches long.With the waistband, cut it an inch or two shorter so you can stretch it when sewing to make the waist snug.
Second you want to pin everything together and carefully try it on. Places you might want to trim: the length, the inseams (where the legs extend from the crotch) and the top, if you want them more low-rise. Just cut and go with the general pattern until you've come up with some flattering booty shorts.
Third you're gonna sew the fronts together and the backs together on those little seams I'm showing you.
Step four will be weird unless you've sewn pants/shorts before. Open up each of those pieces you've just sewn together and lie them flat. Sew the insides of the legs together. Do not sew the front and the front together, and do not sew the back and the back together. If you dont like to pin, suck it up and pin it.
Now you're going to pin the side seams together and try the shorts on. You want to make sure they fit, right? Alright. If they're nice and snug, move on to the next step. If you find that they're kind of weird and baggy on the front or the back seam, go ahead and either dart them or sew the seam in a little bit more.
Step five a is kind of crazy. This is where you sew the trim, which has of course, been folded in half with the sides together already. First off, unpin the side seams. Starting at the top of the waistband on the BACK of the short (which you can see on the right side of the photo near the top), you're going to sew the trim, going around that curve, and you'll end up at your crotch seams. Keep going, you're not done. Sew around the front, and end up back at the corner where the side seam meets your bottom hem (viewable at the left side of the photo.). Cut your thread. You can pin this if you want to but I tend to freehand it.
Step five b involves you matchin up the side seams and pinning them in place. Then you will sew the front, the back, and the trim all together sandwiched. See how the end of the front's seam just disappears underneath the side seam's trim? Thats how you know you did it right. If you turn it inside out, it should look like the photo.
Next you'll put your waistband on. I'm fond of jersey, you might like elastic. If you're using jersey, lay it flat, sew the ends together, and fold the whole thing lengthwise. Sew to the top of the shorts, keeping in mind that you should be tugging a little on the waistband BUT NOT the body of the short to make sure it fits snugly on your bod.
Then, topstitch the entire thing. I topstitched it like I sewed on the trim: starting at the top of the short, down around the crotch, back to the side seam. I then sewed a few stitches onto the loose trim that was kind of flapping around, dropped my needle, changed the angle of the fabric, and sewed right back up to the top of the short.
Here is where I am approaching the other side of the trim on the side seam! Hand-crank your machine so the needle is sunken into the fabric. Then lift up the presser foot, turn it so you'll be sewing the loose edge of the trim, and go to town, as seen below.
Hopefully now you have some sassy shorts to go make the boys drool! Yay!