-a pair of shorts that fit the person you are making the shorts for
-a pencil
-a ruler
-paper (I prefer tissue paper. It comes in big sheets and is easy to pin.)
Turn your shorts inside-out and fold them in half. Line the long side of your shorts up with the edge of your paper. Using your ruler, trace your shorts leaving 1/2" seam allowance. I left 1" at the bottom for the hem and 1 1/2" at the top for the waistband.
And there you go! You have a pattern. It's a good idea to mark the size and top and bottom of your pattern, for future reference.
Now, to use your pattern. Cut two pieces with the long side of your pattern on a fold. Sew the two leg seams. I used a serger because I used knit fabric and wanted to have room to stretch. You can use a regular stitch.
Turn one piece right side out and slip it into the other. Line up the centers and pin. Now sew all the way around.
You should have this.
Now make your waistband. You can serge the edge or fold it over 1/4" for a clean finish. Then fold over 1", pin or press it in place, and then stitch. Don't forget to leave about an inch or so gap so that you can put the elastic in.
For the hem, fold over 1/2", then again, and stitch.
Measure you elastic on your model. Make sure to make it a little tight so that the shorts don't fall down but not too tight. Using a safety pin, pull the elastic through the waistband, being careful not to twist it. Hold on to the opposite end so that you don't loose it. That's never fun. Stitch the two ends of the elastic together and then stitch the gap closed. I like to sew a tag with the size in the gap. If you don't have a tag you can use a ribbon to signify the back of the shorts.
A handy little trick for keeping the elastic from rolling is to stretch the waistband out and sew through all the layers. Make sure you have the gathers evenly distributed before you start or it might not all match up like you want.
And here is my goofy, ratty-haired, Kool-aid smile model. Hey! It's summer. ;)