I needed a new bag and I absolutely love this new bag I made! I followed this tutorial with a few changes to make it just a messenger bag...which in turn made it easier. The tutorial I give here will be almost identical, but all credit goes to Ashley.
For my outer fabric, I used a heavier-weight fabric to make the bag hold shape, and for the lining I used a simple white cotton.
First, I cut out all of my pieces. You will need:
4 pieces cut to be 12 by 22 inches, 2 of outer fabric and 2 of lining, to be made into the main body
2 pieces cut to be 17.5 by 14.5 inches, 1 outer, 1 lining. This will be made into the flap.
2 pieces cut to be 43 by 3 1/4 inches, 1 outer, 1 lining, for the strap
Starting with the 12 by 22 inch pieces, place the two outer pieces right side together and stitch around the one long and two short edges. I used about 1/2 inch seam allowance then zig-zag stiched around the edges.
With the seams still facing out, place your hand inside the bag to open it up. Line up the bottom and side seams and place together to make a corner.
Measure two inches down from the tip and sew a straight line. I did two lines about 1/4 inch apart. Repeat with the other side and then with the lining. Turn the outer fabric right-side-out and leave the inner fabric inside-out.
Next, we will work on the flap. Take the two pieces cut out for the flap and place right sides together. Stitch along the two shorter sides and one longer side and cut the corners (this is to allow the corners to turn out nicely).
Turn the flap right-side-out, iron flat, and stitch around the same 3 edges to finish the look.
Next, take the strap pieces, place right sides together and sew down both long sides. Flip right-side-out, press down, and add a seam like you did with the flap.
Now, you have completed all the parts and it is time to put the bag together.
Line up the center of the straps with the side seams of the main body piece made from outer fabric and pin in place. Center the flap and pin that in place also. You are pinning right sides of the outer fabrics together, with raw edges together.
With the strap and flap pinned onto the outer fabric, place the lining (still inside-out) over it all.
Pin all around the edges, leaving about 6 inches open to allow you to turn the bag right-side-out.
Stitch all the way around the edge, leaving the 6 inches open. Turn the bag right-side-out through this opening.
With the bag right-side-out, pin the opening shut.
Sew all the way around the top, including the pinned portion to finish the bag!