Okay... getting back in the swing of things...
Part 1: fabric requirements and cutting
Part 2: outer bag assembly
Part 3: lining and side seams
And now, to make the flap and put all the bits together.
First of all, take the 3.75"x12" front strap piece, and trim it down to 10" long. Fold it in half lengthwise, and stitch down the long side and one short side. Trim the corners at the stitched end, turn strap right side out, and topstitch two rows of stitching along the three sides.
Baste the flap onto the outer bag, right sides together, with the outer side of the flap on the zipper-pocket side of the bag, making sure that the flap is centered on the middle section of the bag.
Now, we're going to pleat the sides, and attach the shoulder strap hardware.
Cut two more 2.5" sections of the strap that you used for the rectangle ring in part one. Thread each of these through two more rings, and set them aside.
Make a mark 1.25" on each side of the side seams.
Find the 4"x10" strip of coordinating lining fabric. Fold it in half lengthwise, and press. Then open it up, and fold each side inward to meet the center fold line. Press in place, then fold the strip along the original center fold.
Mark the center front and back of the bag and lining.
Turn the outer bag inside out, and place the lining inside it, right sides together (the lining will be right side out), making sure that the pocket side of the lining is on the flap/zipper-pocket side of the outer bag.
Stitch around, leaving an opening on the flap side of the bag, for turning.
Now all we have left to do is the shoulder strap and the snap!