Making underwear is my favorite way to use up scraps of knit fabric that are too big to throw away, but too small to really make anything out of. You can piece together undies from scraps of different colors or patterns to use up any fragment, however small.
The pattern I use is more or less the same as my swimsuit pattern, with a couple of inches taken off the top. Since I didn’t use elastic, I don’t need a seam allowance for these and I also took off a bit around the leg opening. I recommend just trying them on after you’ve sewn up all the seams but before you finish the edges and trimming off a bit at a time until they look right. I add seams here and there to fit the shape of the fabric – I often divide the front in thirds and use a different fabric for the center from the sides, or divide either piece in half and use the same fabric for both.
I finished these on the serger, but you can also use elastic as in the swimsuit tutorial. Or you can use decorative lingerie elastic. To do so, sew the elastic to the fabric with the right sides facing each other (as if you were making an ordinary seam) using a slight zig zag stitch. Then, fold the elastic under, and top stitch using a wider zig zag.