This instructable will walk you through the steps to making a storage bag to strap to your bike. I believe that while this project is relatively simple, dedication to small details in the planning and construction of your bag will result in a great looking bag that matches the quality of many bags on the market.
Here I will divulge what I learned, mistakes that I made, and possible improvements to the art of sewing your own bike bag.
Step 1: Design
My bike frame is extremely tall and I prefer to have my seat positioned more forward which eliminates the possibility for me to simply buy a saddle bag and strap it to the underside of my seat. I also didn't want to have a storage bag on the front side of the seat post so that I might add an additional water bottle holder there.
Because my frame is so tall, there is a large amount of space between the seat post, tire and seat stays. I determined that this would be an ideal spot for my bike bag since it is out of the way and will introduce a minimal amount of additional aerodynamic drag. The only problem I saw with this location is that it will see a lot of spray from the tire during wet riding conditions.
Once you have your location picked out, measure the dimensions and relative angles on the frame. Be careful when considering the width of your bag. Once completed the fabric can easily bulge at the sides and seem much wider than you might have intended. If you race that can be bad, but if the bag is for a cruiser it won't matter that much.
Next consider the opening. How big, long, where and by what means? A zipper is obviously a great choice, but don't forget about velcro or just be creative. Keep in mind that if it somehow comes open during a ride, you REALLY don't want your stuff falling out!
Another key to your design is the attachment mechanism. This has to work in coordination with your opening i.e. they can't effectively overlap. Racers: the smaller and lighter = the better. Cruisers: anything goes. Rope, straps, buckles, magnets*, anything!
* I think it'd be really cool to sew some magnets into the fabric and have it appear to just "float" on the bike!
Fabric is the last consideration. Your type of riding and typical riding conditions will help with this decision. Here are some of the big differences in fabric that you should consider: weight, drag (smoothness), strength, water resistance.
So you have the location, size and shape of your bag picked out. Now you'll take that and break it down for sewing. There are many ways to do this, but here's how I like to do it.
Treat your bag as if it were a rigid object and evaluate each geometric shape. Mine happened to be mostly triangular. Draw them on a sheet of paper. Instead of cutting out each geometric shape and sewing them together, butt some of the common ends together and just fold there. This involves less sewing and therefore a better similarity to the original design. I broke my bag down into two separate pieces. One piece comprised both side triangles and the front (attached to the seat tube) while the other piece was the bottom and rear (opening) face.