You'll need to start out with 2 pieces of fabric, one 10"x9.5" and one 2"x45".
We are going to start with the body of the holder. Fold the piece with right sides together so that the 10" sides are lined up (9.5" side is folded in half).
Using a 1/4" seam allowance, sew down the length and one end of the square with a straight stitch.
When you are done, it should look like this.
Using the end you sewed closed, pinch the corner flat so that the seam on the end bisects the triangle you have made.
Fold the top of the bag over 1/4" (with wrong side out still) and iron.
To iron the strap, start out with the strip of fabric wrong side up. Fold it in half and iron down the length of the strip.
Open the strip you just ironed and fold one edge in to meet the middle crease.
Slide the strap up under the fold you created in the bag.
Repeat this on the other side. I lined it up with the point on the bottom to make sure I was even with the other side.
Use the smallest zig-zag stitch and sew right along the edge of the bag where the strap comes out. Repeat on other side as well.