Sewing machine (Singer Model 626 - thanks Nancy!)
Something to mark the fabric (I used a silver Sharpie. The white pencil from the fabric store didn't work).
Polyester upholstery thread (Joann Fabrics)
Warning: This is the only time I have ever sewed. I have no idea what I'm doing.
Step 1: Create a template for the side panels on paper. Transfer the design to the fabric. Add an inch or two all the way around and cut out the side panel. Obsessively test fit between steps.
Step 2: Add the zipper. I tried to find some basic zipper installation instructions online but I didn't find anything useful. Sew all the way around the edge of the zipper with the front of the zipper facing the back of the fabric. Use an x-acto knife to cut a slit in the fabric to access the zipper. Finally, trim the excess fabric back to the stitching to expose the entire zipper.
Step 3: Next, cut out the top panel and the panels that overlap to connect the bag to the top tube. Attach the top panel and one of the top tube panels to the side panel, all with one seam.
Step 4: Again using one seam, attach the top panel to the other side panel and the other panel the goes over the top tube.You should end up with two separate panels that will meet over the top tube, one on each side.
Step 5: Using one seam, add the down tube panel and velcro to one of the side panels. I used three pieces of 1.5" velcro, but if I built another bag I'd use two pieces of wider velcro. Next, use one seam to connect the opposite side panel, the downtube panel, and three more pieces of velcro. Be sure to get the orientation of the velcro correct.
Step 6: Sew together the point at the front of the bag.
Step 7: Use the same process to create a seat tube panel and sew it between the two side panels. Be sure to include more velcro.
Step 8: Create a panel for the area next to the shock and sew it in. Trim all of the excess material. Flip the whole thing right side out through the zipper.
Step 9: Sew velcro to the top tube panels. Use a zig zag stitch all the way around the top tube flaps to prevent fraying. Once have a chance to try it I'll trim the excess on the seat tube and down tube.
Final results on the bike:
Next up, a seat bag.