Be sure you fabric is lined up on the left side of the measuring line on the ruler not the right side. That little line makes a big difference. Try using a thin thread for piecing, not a 40 weight. 50 or 60 two ply is the best. Pressing seams open will be a big help until you master how to press to one side and not have any distortion. I press most all my seams open. Do not iron your seams or blocks. Press them. Do not move the iron at all on the fabric. Sit and lift the iron.
Also one very important thing. Do not iron on a soft or spongy surface. If the fabric sinks into the ironing surface it will distort into a bowl shape, so little you can't see it but it's there.