Spray starch and press. When sewing the new piece in, put the bias on the bottom. The feed dogs will help work in the fullness. Hope this helps.
I never understood this process. If the feed dogs work in fullness, why do seams that don't require working in fullness come out OK?
My method for working in fullness is to put the "ruffled" piece on the top and use a wooden stick to work in the fullness as it goes under the needle. I just brush across the fabric as it goes into the needle. I can work in quite a bit of fullness this way.
