You've gotten great advice above already. Without seeing the photo, or knowing how you handled the rows that did not match up correctly, that is probably where your greatest waviness comes from. As Prism99 mentioned - you might want to dis-assemble your rows and match up your intersections and re-sew those first before adding any borders. And as a couple of other folks mentioned - put the baggy piece on the bottom, next to your feed dogs to help with the 'easing in' of those larger pieces.
I am of the 'pre-wash; iron/press with starch/steam; square-as-I-go' camp. I find I get better end results this way. I also don't typically use pre-cuts. They are not 'pre-wash friendly'. Sometimes, from what I've heard, they are not cut exactly the same and could also be cut off-grain. But they can be a great place to start your quilting experiences particularly if you're not yet comfortable putting different fabrics together.
Show us a photo if you can and perhaps we can offer more help. But above all - enjoy the process. Quilting is supposed to be fun and relaxing. Your skills will improve over time and you'll find methods that work best for YOU.
ETA - apparently we were posting at the same time. Frankly, I think your top looks fine and those wavy blocks will quilt out nicely. Measure through the center of the quilt top to bottom to find the length of your side borders. Attach them, easing your blocks or stretching your borders as needed. Repeat for the top/bottom borders. Quilt; bind; wash - you'll never see those wavy spots. Good luck.
Last edited by NJ Quilter; 09-04-2015 at 03:55 AM.