Before I quilt a project, whether it be place mats or a quilt, I press it and lay it on the floor. It should lay totally flat, no waves along the edges, or hills in the middle. Fixing those errors aren't always easy. Hills in the middle may require taking apart a row, and making some adjustments. Waves on the edges are a bit easier to fix.
Amandajean at Crazy Mom Quilts has a tip here, showing us how to straighten those rows up so that our quilt top will lay flat before we sandwich it together. Read down to the end of that entry. I like how she points out that your corrections need to be subtle.
Amandajean has some great tutorials. Before you "try this at home", for the first time, her tutorials are some that I would research.