I'd suggest looking on you tube. I've linked a video below for a new sew version. You can investigate getting a gathering foot and trying a straight stitch on the tulle with it using the tightest setting on your top tension and see if it wont gather automatically for you. Or you could get a ruffler foot and see if the ruffler (which is really a pleater) won't help you get the gathers you want.

Here is a link that might be useful: No sew tutu