Cutting and Assembling

From both fabrics, cut:
2 strap pieces, 2 x 30 inches
1 bag piece, 13 x 25 inches

From the interfacing, cut:
1 bag piece, 13 x 25 inches
   

Layer the fabric and interfacing like this on your work table:  

First, set out the interfacing.  Place one piece of the first fabric right side up on the interfacing.  Place one piece of the second fabric wrong side up on top.  Pin these three layers together. 

Sew a 1/2-inch seam around all four sides of the layered stack, leaving a 6-inch wide gap on one side.

Trim the corners and excess fabric.  Then turn the bag right side our and press it flat.

At the opening, fold and press the fabric 1/2-inch so that it matches the sewn edge.  Edge stitch a scant 1/4-inch along the top and bottom edges, sealing the opening and creating a clean edge. 

The body of your bag should look something like this, with different fabrics on either side and clean edges all around.