I have done some work with cuir boulli, and I've researched a bit and found a reasonable way to deal with the stuff.
I start by making a plug form. That means a wooden object the shape of the thing I'm making that has the inside dimensions of the final piece. I heat it and soak paraffin wax into it so it won't soak up water and swell.
Test the leather. The first linked article said to use water at 180 C. That's called "steam." They meant 180 F, and that is a good temperature. Cut half a dozen 2" squares (exactly 2") of the leather you'll be using and poke a small hole in the middle of each. Veg tan is the right stuff. Thread all the squares on a piece of string with knots in it to keep them separated. You'll also want a brick or something else squarish and heavy.
Heat your water to 180F and get a watch with a second hand or get a digital clock with a seconds display. Dunk all the squares in the water while keeping a hold on the end of the string. After 20 seconds pull the first square above the water; after 40 seconds the second square; after 60 seconds the third, and so on. Put them under the brick to cool and dry flat and then check the results. Choose the duration that gives you the texture you want, and note the shrinkage. I found that 40 seconds or so worked well - tough but not brittle. Your leather will vary.
You can also try a lower temperature (say 140F) for a longer time and see how that works with your leather.
Now you can sew your piece, wet it, slide it over your form, and then dunk it for the right duration. It will shrink to a tight slip fit over the form, so the wax on the form is necessary.
