Mark the ruffle fabric every 1/4 of it's lengthy. Do a long running stitch. Leave the thread ends long. Tie a knot in the thread at one end. Grab on long thread on the other end and start pulling. Keep pulling and adjusting until the gathers are even the length of the fabric. Using the marking you made, pin it to the edge of the quilt and sew.
Wow! That reads much more complicated than it is. Maybe someone with better teaching skills than me will chime in.