As in my post http://tiramisue-suesblog.blogspot.com/2010/03/never-fail-mitred-corner-blog.html the glue stick is your friend!!
Here are the pictures. Place your two strips like so, right sides together.
Fold the top one back on itself until you have the 45 degree angle.
Place some glue on the folded piece (the bit that will be removed). Stay away from the 1/4inch seam allowance.