Once you have stitched two strips together, set the stitches by first pressing the stitches, then proceed to press the seam to one side or open. It also helps to press along a straight line. Think about using a pigma pen and draw a straight line on your ironing surface and making one edge of your strips align it.
When cutting your fabrics, it is best to cut all in the same direction, i.e., either from selvage to selvage or lengthwise along the selvage. There is different stretch for each and may also be the cause of "wavy".
Suzanne