I've been working on a new bloomer/short shape (not sure what to call it yet) and instead of knicker elastic I've been using bias binding threaded with elastic. It gives the knicker a cute retro look, here's how you do it....
1. First, stitch the front and back together at the crotch and side seams, and then neaten. With right sides together stitch the bias binding (I've been using a soft satin bias 11mm wide) around one leg opening. Stitch just to the right of the fold rather than right on the fold.
2. When you've almost stitched the binding the entire way around stop leaving a gap of about 5cm so you can make a join in your bias binding.
3. With right sides together make a join in your binding. Check its correct by pinning first.
4. Trim away the excess binding, press the seam open and then stitch this section to the leg opening as before.
5. Flip the bias binding to the wrong side, press and then stitch along the free edge leaving a small opening to insert the elastic.
6. Repeat steps 1-5 for the other leg and waist and then give the knicker a good press.
7. Thread elastic (I've been using a soft 6mm wide elastic) through the openings you've left using a small safety pin. Take care not to twist the elastic. The amount of elastic you need will depend on your size, a general rule is the total circumference of the fabric x .66 Once confirmed overlap the elastic by a couple of cm's and using a zig-zag stitch, stitch back and forth a few times on the overlapped section. Using a straight stitch, stitch the opening in the bias binding closed....and voilà!