Seam equal width strips of the two binding fabrics, each twice the width of the finished binding.
Press seam one way, then the other way, and you should be able to fold the two strips wrong sides together and press again. Open up and fold each edge to about 1/4" from the inside center seam, just about lining up with the seam allowance; press.
You now have your binding ready to sew on. Having the seam pressed ahead of time helps keep it straight.