We used a single ruffle and made them out of knit. But with the following tutorial you can make as many ruffles as you'd like and use knit, tulle, or woven. (You'll need to finish the raw edges if using a woven though.)
Fabric for the shorts + 4" for the ruffle strips (single ruffle)
Low tack tape such as painters or masking tape
The ruffle strips are 1 1/2 times the leg opening in width and 2" in length, each.
2. Gather the ruffle strips. We use our machine to gather. Set the top tension at a higher than normal number (the dial near the top, probably set on 4 or 5 normally). Set the stitch length at the longest length.
Lay the ruffle on top and tape it down (above).
Adjust the ruffle to the same length as the shorts leg. (below)
Remove tape, and do the same for the second leg.
5. Sew the shorts together following your pattern directions.
6. Match the ruffle seam up to the shorts inseam. Lay the ruffle on top of the bottom edge of the shorts leg. The gathering stitch will be about 1/4" to 1/2" from the shorts edge. (above)
The ruffle circle should fit the pant leg with minor adjustments in the gathers, if any.
Stitch the ruffle on by sewing on top of the gathering stitch and regularly checking to be sure the ruffle is sitting in the proper spot on top of the shorts leg. (below)
Repeat for the other leg.
If you want a second row of ruffles, simply place the second ruffles at a good spot above the first one and repeat this step.