I would put the snap tape on first. Fold up the hem and make sure you fold in the edge so you are sewing on the folded edge to enclose the raw edge. Thread the elastic thru and stitch into the end openings to sew them shut. The snap should be just above the hem a little bit. I got so tired of snapping all those little snaps with the legs going a mile a minute, that I put velcro in one time. Guess it worked as well as the snaps. They all come open sometimes. What are you making the romper out of? If it's heavier, like corduroy then you will need a wider elastic to compensate for the heavy fabric being bunch up. Good luck.