Next part!
I made a hole in the shorter part of the strap where the buckle would need to fit through, using this hammer and punch. I had to cut it to be slightly longer later due to the design of the buckle:
I cut a small piece of leather to be the thing that you tuck the watch strap into, and sewed a piece of thread into each end. I was going to do the full loop with the leather, but I decided it would be too fiddly to attach, so I went for leather for the visible part and thread for the part that you can't really see:
I sewed the buckle onto the short strap, attaching the little loop thing in the process. I used the pins to hold the leather in place while I was sewing. In order to get the pins and the needle to go through both pieces of leather, I gently encouraged them with a light tap of the hammer:
I sewed the other end of the watch strap onto the watch in the same way, and then I did the same for the long watch strap: