I saw this method demonstrated on YouTube, sorry to say I lost the link. However it is so simple and effective I thought to pass it on. Here it is, set your tension to #8 and stitch length to #5. If you cannot do either of these things then this won't work for you. However once things are set just sew along the edge. Do not back stitch and be sure to leave at least 4inch thread tales on either end. The gathers are easily then adjusted as needed. I recently needed to add ruffles to a curved hem on aprons, this worked great. I just pinned at the center and ends of the seam and adjusted the ruffles as I pinned along the perimeter. Of course this is basically a basting stitch so you do need to over sew the gathers with a regular stitch length and tension. I have a ruffling-foot and a gathering foot but I found this a very easy solution to how to get the gathers to fit in the seam you are trying to match it with. If anyone has a link to the You Tube video on the gathering technique please post it so they can get credit for this idea.
Another tip I found helpful is if you have a serger and can do a rolled hem stitch, then I just finish the bottom edge rather than hem it. I use all four threads, widest seam width setting and a short stitch setting. Then I get out some bright variegated serger threads and finish off the bottom edge this way. It adds a pop of color, and gives the edge a clean hemmed finish. It adds a decorative edge to the ruffle. It is also a great and easy way to do a quick finish to kitchen towels and wash cloths.