Baste the front waist seam, then using a pressing cloth so as not to melt the PUL, press this seam open. You could skip this step but I do it because it gives me a nice neat straight folded edge later when topstitching.
Stitch the rest of the way around to join the two pieces. I used a 3 stitch length. You don't want too small of a stitch with the PUL.
Rolling the seams between your finger and thumb before topstitching helps to get the seams right to the edge.
I'm looking at changing the soaker pattern as well. The prototype one is just a big rectangle, but I think something that is wider on the back side will better contain messes and make them easier to deal with.