When a “bird’s nest” of thread appears, the likely culprit is your upper thread tension. If loops appear on the underside of the fabric, the upper thread tension may be too loose. If knots appear on top of the fabric, the upper tension may be too tight.
Fixing fabric grain - learned these methods long ago in school. Not only does it help your finished product, it reduces the amount of excess trim when squaring fabric up to cut. I wish I learnt this at school!