I've just finished making up a sample for a new pant shape I'm working on. Made from a heavy weight jersey I used 32mm wide elastic for the waist which is overlocked on and then rolled. It's really quick and looks great, here's how you do it....
1. Cut a length of elastic for the waist and join the ends together using your overlocker.
2. VERY IMPORTANT disengage the cutting blade on your overlocker!
3. Divide the elastic evenly into four and mark with chalk, do the same for the pant waist. With right sides together pin the elastic to the pant at these 4 points, 4 pins only. The top edge of the elastic and the pant should be in line.
4. This next bit may take some practice and a few muscles, overlock the elastic and the pant together around the top edge stretching the elastic to fit. The jersey underneath should not be stretched.
5. Roll once and then again so the elastic is hidden. Secure the roll at 2 points, usually the side seams but in my case the centre front and centre back, by stitching in the ditch through all layers.
.....Voilà! the finished waist.