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.
12 Sewing Hacks Every Crafty Person Should Know (But Probably Don‘t)
Perfect hem line - draw a line on card stock, 1 inch in from edge of card stock. Place the card stock on fabric, with line parallel to fabric edge. Fold fabric over card stock, aligning fabric edge with line; press with an iron.