So, here is the sizing I cut my fabric up in. The fabric by the way was a thick canvas with a thick plastic backing. I used the plastic backing as the lining of the bag.
You can add a 1/4" seam around it, I didn't. So, my actual bag came out a little smaller than the measurements here. I pinned the front piece and the side pieces canvas side's together on my first stitching and sewed them together.
I then turned this piece right side out and sewed the bottom of the back piece on.
Now, this is going to sound confusing, but I turned the bag right side out again and sewed the two sides of the back piece onto the bag.
After this, I cleaned up the flaps edges and folded them under and sewed.
|bag finished without straps|
For the strap I folded it in half and ironed in on the canvas side....and yes it was ok to iron, even with the other side being plastic. Then I folded it again (quartered it) so that the sides were touching in the middle and ironed it again. Keeping the sides touching in the middle I folding the whole thing in half and sewed along the edge.
I used gromets to attach the strap to the bag. Then using the fabric transfer paper I transfered my image (google image search) onto the front of the flap.