If your top is already quilted, you will not be able to pull it inside out as butterflywing instructed.
You would have to sew the ruffle to the the top, and then add a facing on the back. It's not hard
Ruffle:
To determine the width:if you want it 2" wide, cut it 5" wide and fold it in half, matching raw edges, wrong sides together, and you will have a 1/2" seam allowance.
To determine the length with double fullness, measure all 4 sides of your quilt and cut your ruffle twice that length, plus 1", sew the ends together to make a circle. Fold the width in half, wrong sides together, and press raw edges together.
Using strong thread, sew two basting stitches along the raw edge fo your ruffle, one 3/8" fron the edge, the second one 5/8" from the edge, to not back stitch. Leave at least a 6" tail of thread on each end. You will use these tails to pull and create gathers. You can break this up and stitch one side at a time. Be sure to leave the basting tails long enough to pull, for each side of the quilt.
Tie the two basting threads together at both ends on the front and back. A double basting is good for 2 reasons. 1. if one breaks you still have one to pull. 2. Sewing between the basting stitches insures that proper alignment of the ruffle.
Mark the quilt every 3" and mark the ruffle every 6" Pin the ruffle to the quilt, matching marks. Then carefully pull the double basting threads to gather the ruffle evenly, all raw edges together and sew through all layers using a scant 1/2" seam allowance.
Facing: Cut a bias strip the length of all 4 sides of your quilt plus a couple of inches to join or overlap where the ends meet.
If you are using a half inch seam allowance cut your bias strip 2.25" wide. Fold it in half, and press lengthwise. Note: bias is not necessary, but preferable. Be careful not to stretch the bias as you press and attach it.
Then attach the (right sides together) folded facing, raw edges together. Use a full 1/2" seam allowance or a hair bigger, to completely hide the ruffle stitches.
Flip back the folded edge of your facing and hand stitch it. Miter the corners by tucking the extra fabric at a 45 degree angle inside as you sew the corners down. Be sure to knot your hand stitches often :)
Remove all basting stitches... and you are done. Now take a picture and share it with your friends.