I often have the problem you describe. For me, not cutting the fabric on the straight of grain seems to trigger all this waviness. I tried starching the fabric before I cut it, but if I don't get the fabric perfectly straight before I press it, it gets cut off grain resulting in waviness. Even if I cut fabric perfectly, once the fabric gets wetted with the starch, it stretches and moves around. So I don't worry about it any more. I now cut the fabric, stitch it in place, then spray it with Best Press and press, then trim it straight. Or, if I'm doing a pattern like Log Cabin, I simply cut the outside strips a little larger, press, then trim it to size. I like Best Press better than starch because it seems that when I get a crease in the fabric it is easier to press the crease out with Best Press.