I've based this vaguely on my Carradice and the cheapest bag i could lay my hands on. The bag is basically big enough to carry my commute gear, a shirt, underwear, travel towel and sandwiches.
Step 1: Choose a Bag
After you've chosen a bag its probably best to wax it to provide a degree of weather proofing. This was the first time I tried to wax any thing and made a hash off it as you can see from the colour of the bag. I forgot to take a picture when I got the bag but the colour variation is visible from the strap.
Step 2: Add the Dowel
I've used 15mm as it was the thinnest that i could find that didn't flex much under load, a little flex is likely to be ok but too much and the dowel might snap.
Cut the dowel to the length of the bag.
Then drill two pilot holes toward the end of the dowel, not too near the edge or it might split.
Now take two small screws and screw them threw the fabric of the bag to the dowel resting on the inside.
Step 3: Prepare the Leather Reinforcement
Cut a small length of leather this one is about 10cm.
You want it to run about 4 or 5cm across the top of the fold to give the bag some structure.
The length down the back is less important a couple of cm should be sufficient.
Now you need to mark the holes where the thread will go.
I've used about 6 stitches to an inch 8 would probably be better.
Mark up and use an awl to punch the holes, this is book binders awl but i don't think it was really thick enough for the thread and needle a saddlers awl would have been better.
Having said that, I used a mallet to put the holes through then put the pilot hole over a drill hole over my sacrificial timber and forced the awl through to open the hoe out.
finally cut a small square out for the straps to pass through.
Step 4: Stich on the Leather Reinforcement
You need to measure the distance between your saddle rails or bag loops if you have them, then place the leather about that distance apart whilst being central on the bag.
So stitch them up using good quality linen thread, not cotton as it can rot and not synthetic as it can rip the leather.
Using one length of thread go round and stitch trough all the holes, then on the holes where you went down come up. There is probably a proper name for this but i don't know it...
Once they are both secured cut out a section of the canvas.
My stitching isn't brilliant but this should hole well enough, these aren't load bearing they are mostly decorative but they stop the canvas from tearing and add some structure to the flap.
Step 5: Rivet on the Bottom Strap Holder
Cut another couple of lengths of leather and punch a hole in each corner.
I used a hole punch and a mallet but you could use a bigger awl or a drill bit.
Put the rivet trough the first piece of leather inside the bag then through the canvas and finally into the second peice of leather.
Using a rivet set, set the rivet with a mallet.
Make sure its nice and secure, again this doesn't take much weight but only stabilizes side loads.
Step 6: Mounting
Pass some toe clip straps out the bag and through the loops on your saddle then back in to the bag and secure them around the dowel. the retaining strap goes round your seat post or frame.
I've used a Carradice SQR here because my saddle doesn't have loops and I had one spare. Its also easier to see what is going on.