Here's what I do. I take them to my ironing board and place a pin straight down through both pieces exactly at the seam allowance. I can check the match by lifting up the seam allowance or the pieces. Then I place a dot of elmers white school glue in the seam allowance near the pin and iron it dry. The glue acts as a "pin" so the layers can't shift while I am sewing. Hope this helps!