I used to do more ruffles and fancy trims around quilts when I first started, gave them up for the standard french double fold bias... That is, you put the two raw edges together and stitch on, then flip over and hand finish.
Anyway, I found the purchased trims or making my own to work the best, basically I just make it all into standard double fold bias. That is where you press in half, then fold the raw edges into the middle and press again.
So, next thing is don't be stingy on how much fabric you use. Twice is not enough, you really need 2.5-3 times plus for a good gather or ruffle. If you are using quilting fabric it needs to be pretty substantial depth, at least 2" finished or it will just be a big heavy lump. It will still be a heavy finish if using double thickness but can be rather nice for a deep 4" or so ruffle.
If using a lace or purchased single thickness trim, gather first and then case into the binding. You would have one long raw side plus one folded side of the bias tape, stitch with a zig zag the quarter inch seam allowance with the raw edges together, the folded edge would be sewn to the top.
Figured it would be easier to make an ugly example than explain it well...
First shot shows the bias pinned to either side of the ruffle, you can carefully pin both sides at the same time or do once each for a sturdier finish. I enhanced my ironed press line with pencil so they can be seen.
Second shot shows the inner fold.
Third shot shows the encased binding you can miter like regular binding or I usually gently curved (like around a quarter). Depending on what you used for the ruffle depends a bit on how you finish that seam.