The way I sew fabric strips to make them longer is to lay one right side up horizontal, lay the second wrong side up vertical on horizontal strip end, leaving about 1/2" to the end. I then mark a diagonal line from corner to corner and sew on the line. Open the strips to be one long strip. Trim the seam to 1/4".
Found better explanation with pictures.
http://www.mccallsquilting.com/artic...binding_strips