In short, I needed to soak the leather and then force as much sand into it as possible to get it stretched into a bottle shape. Soaking it was easy. Happily, it took exactly as long as it took me to go grab a shower. Coincidence? Surely not! (Note the kind assistance of Darcy, our local helpful housecat.)
After that was done, the real work started. This took the following tools:
- A bag of sand.
- A funnel and scoop.
- A wooden spoon.
- A dowel.
- A hammer.
Eventually, I settled on a kind of system to maximize the amount of sand I could force into the flask, while minimizing what was scattered around the counter. I recommend starting by blowing a puff of air into the bottle to force the leather apart and give the sand somewhere to go. Then insert the funnel and pour in the sand. When it backs up, start using the handle of the wooden spoon to force more through the funnel.
That will eventually not work out anymore; start using the spoon inside the flask itself. Force it down, remove the spoon, repeat. Add sand as necessary. When it starts to fill up, use the dowel and the hammer. Don't be gentle here - I found that the leather was super resilient, stretched a great deal, and pounding as much sand in as I could was really entertaining. Seriously, you will use more sand than you think will fit.
So that's all done. Now I'm just letting it dry, which will take forever. Once the leather's dry, I'll probably whip a little extra thread from that top stitch up on each side of the neck, to keep it from gapping more, but that's not a big deal. Then I need to get some beeswax. The plan now is to melt the beeswax, pour it inside, let it soak in, pour it out, repeat a bit, and to call it a day! I'm excited.