One way is to mark the sewing line on each piece. Another is to simply mark the corners on each piece. To do this you can make or purchase a template.
If you make your own, it's probably best to use template plastic rather than cardboard; cardboard can become inaccurate with use.
Marking is usually best done with a mechanical pencil (.05) that has a soft lead.
Also, how are you piecing? When hand piecing, you do not want to make long strips the way you would with machine piecing. When hand piecing you can work from the middle out. For example, sew 2 squares together, then sew on a 3rd square, then a 4th square, then a 5th square, etc. If you simply add on one piece at a time it is much easier to get corners to match because you are always starting in a corner.
Also, when hand piecing, do not sew all the way to the edge of the fabric. Stop at the corner and backstitch. That way all corners are free to be pressed in any direction.