Contgrats on your first quilt!!
You can also get some light weight knit fusible interfacing. Tuck it into the quilt, with the fusible side up, then pin the fabric in place, as the other posts directed. USE A PIECE OF COOKING PARCHMENT< OR A NON STICK PRESSING SHEET on top of your quilt, then press according to the instructions for that interfacing. This will stablize the fabric. I would then stitch over it. Be sure to put something between the iron and the quilt or you might get stuff on your iron.
I would do sort of a wiggle stitch over the seams if you think that there may be additional areas that had a slightly too small seam. Just a wave that crossed back and forth over the seams.