The prototype was made in a leftover lightweight home dec fabric. The second used denim recycled from an old pair of jeans and stenciled with a combination of Setacolor and Lumiere fabric paints. The third, in a green linen-cotton blend, was also stenciled with the same fabric paints. All are lined with assorted silky lining scraps and have a top zipper.
These are the finished dimensions. I added ½" seam allowances, so the cut sizes are 5½" x 6½" for the sides, 1¾" width for the gusset. The cut width of the strap is 2½" (twice ¾" for the front and the back of the strap, plus ½" seam allowance on each side).
Cut the same size front, back, pocket and gusset pieces in lining fabric.
Next, construct your pocket. Sew the upper edge of the pocket and the lining, right sides together, leaving about an inch gap through which the pocket will be turned right side out. Now pull the lining down (rights sides still together) so that there is at least 1/2" of the fashion fabric above it. Cut off the excess lining at the bottom of the pocket. Sew the sides and bottom of the pocket with a seam slightly larger than 1/2", so your pocket won't quite come to the edges of your front. I also slightly rounded the bottom corners; if you do so, be sure to clip the curves. Turn it right side out through the gap you left, carefully poking out your corners. Press. Stitch it to the front, stitching close to the edge. (You see my pocket interior turned out a little less than stellar, as I was not careful enough turning out the corners.)
To give the bag more body, I fused a firm interfacing to the front and back. This was something I had in my stash that I had used for another project, it may be Peltex, but I'm not sure; it is about 1/8" thick. Use whatever gives your fabric the body and firmness that seems appropriate. I cut the pieces slightly less than the finished dimensions and fused them on with Pellon Wonder Web.
Now turn under the 1/2" seam allowance at the top of the front and back pieces and press. Stitching close to the edge, sew the front and back pieces on to the zipper, 3/4" apart (the same measurement as the gusset). The excess front and back sections of the zipper will simply tuck to the inside.
If the gusset was cut as two pieces, seam them together. Staystitch both sides of the gusset where it will turn at the corners. To determine this, pin the center of the gusset to the center of the front.
Now comes a slightly more difficult part. Stitch the other side of the gusset to the back in the same manner, making sure the zipper is partially unzipped . Turn the bag right side out. If you're using a stiff-ish fabric, you should turn your corners carefully and press. (However, you will be turning the bag back wrong side out again.)
At this point your zipper ends will be out. Poke them back inside the bag. As best you can hand stitch the closed end of the zipper to your gusset. Now turn the bag wrong side out again. Hand stitch the excess part of open end of the zipper together up to where it meets the bag.
Now construct your lining by sewing the gusset to the front and back pieces just as you did with the fashion fabric. Turn under the 1/2" seam allowance at the top and baste. Pull it up over the wrong-side out bag and hand stitch the front and back (but not the gussets) to the zipper tape. The excess end of the zipper should be between the lining and the fashion fabric.
Turn the back right side out again. All that's left is sewing the strap and attaching it. On the first two bags, I folded my strap right sides together, stitched the seam and turned the strap, making the seam go down the middle of the inside of the strap. Because the strap was narrow and my fabrics fairly heavy, turning it was very difficult to do. On the third bag, I pressed under the seam allowances so that they ran along one side and then topstiched them together. This was much easier and actually looked better. To attach the strap, first turn under about 1/8" to 1/4" at the gusset ends and press; baste if necessary. Wrap the gusset ends around the ends of the strap and pin. Then turn in the raw edges down the middle of the ends of the gusset and hand stitch them as neatly as possible. On the other side machine stitch around the entire gusset end as shown. I also tried using D-rings to attach the strap; while this was easier, I didn't like the way it looked, but it's an option.
You're all finished. Pop some stuff into your little bag, put it on and off you go!