1, Here is the fitting I start with. I have already marked and drilled the holes, my brass rivets are 2 mm in diameter so I drill the holes with a 2 mm drill bit. Be careful to get the holes straight through both the brass and leather and insert the rivet.
2, Here is the tools for the job. A wire cutter, a small ball peen hammer (100 g), some brass rivets (2 mm) and a broken off saw blade (more of this later).
3, You will need a hard and flat surface to peen against. I use a block of steel, but an anvil or a piece of railroad track works just as well.
4, Put a rag or a towel between the peening block and the fittings. This will prevent scratches and dirt on the leather.
5, Now, take your broken off sawblade or anything else that is 1 mm thick and has a hole in the end. Carefully place the piece over the rivet.
6, Use the wire cutter to snip off the rest of the rivet, leaving a short piece.
7, This is what it should look like. You could do all rivets like this before staring to peen or cut and peen and the moving to the next one. Be careful so the rivets don't fall out, it's a hassle to put them back in when they are so short.
8, Use a small ball peen hammer and carefully tap the rivet directly from the top. Use the saw blade to align the taps on the rivet. Do not strike to hard, gentle taps is the key to getting a good peen. The hammer should be doing all the work so just let it fall against the rivet.
9, You should have something like this. A flat rivet head that has been mushroomed over the metal.
10, Use the other side of the ball peen hammer (the ball side) to gently shape the flat rivet head. I usually tries for a round head on this side as well.
After this step the rivet is finished. Use some time to smooth the rivet out and then give it a good polish like you would do with the rest of the brass. I peen the rivets before any polishing and this helps to buff out any marks left by the hammer and tools.
If you don't have rivets with a finished head just use a piece of wire and make the two sides like above.
I hope this is helpful!