My cutting is never perfect so don't get frustrated. If your cut pieces are not cut to match perfect, then iron the piece to a freezer paper cut to exact size the square should be. Then you can draw a 1/4 seam line and follow the seam line not the fabric edge. This is a good way square up blocks too. For lightweight cotton a size 50 thread will be fine. The universal sized needle will work with the size 50 thread.