Take your fabric and fold it in half with the pretty sides facing each other and the fold at the top of your work surface. Measure across the narrow width of your foam and add 2″ to that number. (Add 3″ if you are using 2″ foam.) Use a straight edge to draw a line down from the fold to the bottom of the fabric. Take the number you worked out for your foam and measure that far from the first line. Draw the line down from the fold to the bottom of the fabric. Pin the fabric together down the lines. Hand or machine sew down the lines.