I considered doing a PDF of the pattern pieces, but they are mostly too big to fit on letter sized paper, and pretty easy to make yourself if you have a ruler, a 6.5 inch circle, and a coffee cup on hand. The coffee cup is to hold some hot coffee. Obviously.
Click on the image below to see or print a larger diagram for measurements. This is not to scale.
YARDAGE: Assuming the width is 45 inches, you'll need about a yard each of the outside and the liner, plus your interfacing if you are using it.
THE FINISHED BAG DIMENSIONS, EXCLUDING THE STRAP:
about 12 inches x 12 inches x 3 inches wide at the bottom.
1/4 INCH SEAM ALLOWANCE IS INCLUDED IN THE MEASUREMENTS. If you are more comfortable with a larger seam allowance, be sure to add that when you draw your pattern.
Draw and cut out all your paper pattern pieces. Draw a rectangle for the flap, then use a 6.5 inch circle to trace the curved corners on the flap. I have an obliging embroidery hoop that is just the right size.
Now trace and cut out your fabric.
NOTE: If your main fabric is not a heavy, purse-worthy weight you should consider adding a fairly stiff fusible interfacing on the strap, outside body, and outside flap pieces. Cut out the interfacing in the same size as those pieces, and iron it on according to the manufacturer's instructions. But remember, you are going to have to turn this bag through a 5 inch hole, so the interfacing shouldn't be too stiff for that.
NOTE YET AGAIN: Even if I don't specifically mention it below, you'll be of course pressing and finishing your seams in your preferred method for each step.
1) Strap. Press it in half, right sides together, longwise so you have a 2 x 45 inch piece. Sew it closed along the long edge. Turn it right side out (use a hugemongous safety pin like I do to help with this. Or if you have a fancy tube-turner, by all means, get that thing in action). Press, then edgestitch down both sides. So now you have a lovely strap with raw ends.
2) Outside Pocket. Align the outside pocket pieces right sides together and sew all around, leaving a couple inches open on the bottom side to turn it. Snip the corners. Turn and press. Pin it to the right side of the body piece, centered side to side and about 3 inches down from what will be the top of the bag. Edgestitch it on the sides and bottom, leaving the top open (obviously) and backstitching at the beginning and end to reinforce.
3) Inside Pocket. Now do the same thing to assemble your inside pocket, but pin it to the right side of the body lining fabric, centered from side to side and about 4 inches down from what will be the top of the bag. Edgestitch it on the sides and bottom, leaving the top open and backstitching at the beginning and end to reinforce. Then stitch again down the center from top to bottom to create two little pockets.
4) Flap. Now is the time to do all your embellishing of the flap on the outside piece BEFORE YOU SEW THE FLAP TOGETHER.
(Musical interlude while this creative wave crashes over you.)
Once your outside flap piece is appliqued or pieced or whatevered, you are ready to proceed.
5) Align the (fabulous) flap outside and lining pieces right sides together. Sew all around, leaving a 3 or 4 inch gap in the middle of the top edge for turning and backstitching at beginning and end. Snip the corners and clip the curves. Turn it and press, turning under the unsewn seam. Edgestitch all the way around. Now you have a lovely finished, but disembodied, bag flap.
6) Lay the flap down right side up on the right side of the body piece (same end as where your pocket is), centered side to side and about 1.5 inches from the top edge. Edgestitch following your previous stitches, then stitch again about a quarter inch from the edge, reinforcing at each end.
7) The Body. Fold the body piece in half, right sides together, so you have a 14 x 12.5 inch rectangle, with the flap tucked inside and out of the way. Pin and sew up both sides.
Finger press or press the bag flat so you have a line that goes down the center of the bottom of the bag. It's just a guideline. You are going to square the corners so your bag has a flat bottom. Heh. Pick it up, bring the side seams together at the top and lay it back down with a corner pulled out and flattened so it looks like a triangle (see drawing below), with the side seam going down the middle of the triangle and lined up with the guideline you pressed in the center of the bottom of the bag. Pin it to keep all that in place and use a ruler to draw an air soluble line at 3 inches across the width, then sew it securely. Repeat that to square the other corner. You can trim off the excess corner or fold it under and tack it to add body to the bottom of the bag.
Now repeat this step with your lining piece, folding it, sewing it, and squaring the corners.
8) Now is the time to test the strap length. Turn the body right side out with the flap in place. Pin the strap in place on both sides of the bag, centered with the side seams, and with a 3/4 inch overlap. Sling it over your shoulder diagonally across your body. Sashay to a mirror and make any adjustments in strap length so the bag falls in a comfortable place on your opposite hip. Unpin and trim your strap to the new length, being sure to include the 3/4 inch overlap. Now baste your strap on the right side of the body upside down, centered with the side seams, and with the ends sticking out over the top 1/4 inch. (Make sure the strap is not twisted! Because that would be really embarrassing later. Ahem.)
9) Now you need to put the body and the lining right sides together. So turn the lining right side in, and then stuff the body inside that one so that their right sides are facing each other. I like my inside pocket to be on the same side of the bag as the outside pocket, but that is just me. The flap and strap will be stuffed down in between the two layers, so just tuck them in and smooth them down the inside so they stay out of the way of the stitching. Pin the body and lining together at the top, lining up the side seams and the raw edges. The strap ends will be sticking up out of both sides.
10) Sew the body and lining together, reinforcing over the straps, and leaving about 4 or 5 inches open in the center of the flap side for turning. Make sure you backstitch at the beginning and end really well. Turning this stiff fabric puts a lot of stress on your backstitches. Turn your creation and then stuff the lining down inside the bag. Wow. Now it looks like a real bag!
11) Press the seam around the top, rolling it between your fingers to get it as close to the stitching as you can. Press under the seam allowance on the opening. Edgestitch all the way around the top of the bag (don't catch the flap in your stitches. Ahem.). You'll probably need to press the rest of the bag too, because after wrestling it through the hole it tends to look crumpled all over.
Shazam! You are done. Look at that fabulous bag you just made. Bravo!