First Step: Don't cut anything! :)
You'll be ruffling right off the chiffon rolls onto the lower tier of the petticoat.
Now, ideally you should not sew up the back seam of the petticoat's lower tier until you've finished ruffling! That way you can always trim it or add to it if necessary. The idea is to keep your fabrics as open to alteration as possible so you don't run out of room or end up with way too much extra.
Look for the PROCEDURE in the next post.