Here's what the finished bag looks like. You can make it any dimensions you want, but mine is 13" long, 10" wide, and 2 1/2" deep. The strap is 40" long and the flap is 7 1/2" long. Use these measurements (plus 1" for seam allowances) when cutting out your fabric, or create your own. I've also made this pattern very small (about 6" x 4" x 1/2") and it came out great.
Cut out your fabric. The photo shows the pieces you need. From top to bottom: cut 1 "flap" of fabric and 1 of lining; cut 2 "front and back" of fabric and 2 of lining; cut 3 "gusset" sections from fabric and 3 from lining (two gussets are the same size, and one is slightly smaller, for the bottom of the bag); cut 1 "strap" from fabric and 1 from lining. The dimensions of the pieces should be the finished dimensions plus 1" for seam allowances.
If you're using interfacing, sew or iron it on first to each of the pattern pieces. I ironed mine on to the lining, since I used kind of a thin cotton for the lining and it needed some toughening up.
Next, sew the 3 gusset pieces together at the short ends, with one long section on each side of the shorter section. You should have one long strip of fabric with two seams. Press the seams open.
Next, pin the gusset to one of the "front and back" pieces (which are identical at this point). The photo shows what this should look like.
When pinning the gusset, make sure to pin the corners first: the way to do this is to clip the gusset seam a bit and pin it to the fabric as shown in the photo.
Pin the rest of the gusset on and stitch; trim the seam and clip the corners. Stitch the gusset onto the other "front and back" section in the same way. The photo shows what it looks like when both sections are pinned/stitched to the gusset - you can see the bag starting to take shape.
Turn the bag rightside out when both sides are stitched, and press seams.
Next you're going to make the flap that attaches onto the front of your bag. First, attach one half of a magnetic snap to the lining. Pin the fabric and lining flap sections together and sew around three edges, leaving one long edge open. Trim seam, turn rightside out, and press. Sew optional trim around the edges. The photo shows what it looks like when finished.
Attach other half of magnetic snap to bag front.
Pin the flap to the back of the bag, right sides together.
Pin strap and lining sections together and stitch along long edges. Trim seam, turn rightside out, and press. Topstitch along both edges. Pin the strap to the gusset sections, right sides together.
I made two interior pockets - cut four rectangles from the lining material. Pin two pieces together for each pocket, sew around the edges, and leave a small opening for turning. Trim seams, turn, and press. Pin the pockets to the "front and back" lining sections. I made one a pen and pencil holder (in photo on right) and one is a regular pocket with a Velcro closure (in photo on left).
Topstitch around edges of each pocket; topstitch in 1-1/4" rows for pen holder.
Prepare gusset lining sections same as for main bag. The only difference is that you should leave an opening in one side to turn the bag rightside out. The photo shows the bag lining stitched together with the opening in the bottom.
Pin lining and bag together at top edges, with bag inside lining and right sides together. Stitch top edges together, trim seam, and turn rightside out through hole in bottom. The photo shows the bag after turning.
Press the top edges of bag, and sew up the hole in the lining. Push the lining down into the bag, and press the top edge again. Topstitch around top edge of bag, and voila!