When you roll the seam between your fingers it will for the most part fall into place. It might help to have some grippy gloves on when you do this or dampen your fingers a bit so they grip the fabric. If you top stitch close to the edge you'll get a finished look and it will hold the seam in place. You can either pin near the seam of press to hold in place before you topstitch. The topstitching if done close enough to the edge will also take care of the opening where you birthed the placemat.