One of my big problems is ironing instead of pressing. I'm quilting a quilt now that had a lot of bias edges. I had used sizing instead of starch (now I'm returning to using starch) and looking back I know I didn't handle the pieces with care so they became very wonky. I'm surprised the quilt came together as well as it did. Perhaps starch would help and be careful with pressing, along with practice, practice, practice. Just don't give up!