You pay the tinker, (about one upgrade's worth), and basically refuse to leave as you watch the item take shape. Sessha heads out, to do something, and basically just leaves it between the tinker and the two of you to discover what went on.
This is slightly disappointing when you get to just watch someone bang metal for an hour or so, while talking about obscure tinker politics. Blah blah blah..
..eventually you run out of cool stuff to look at, (Where are the gearwheels? Where are the bubbling potions? Where are the inexplicably flaming metalworking torches.) and head out of the tinker's small shop, out into the night air.
As promised, it takes three days, and at the end of it you've shattered a set of metal shutters on someone's house, caught a chicken, set a pig on fire, picked a fight with Stinky, the local Top Kid, and did a footrace around the lake. The road orcs have been taken out by a couple parties of locals, and Sessha has been visiting the witch for purposes unknown. Uncle is still tinkering with his own thaumulator, so you mostly leave him alone so as to not feel icky.
It's rubber and metal, and it's nothing more than a big tank of air hooked up to some kinda backpack thing, with a fancy iron helmet that covers your entire head. Putting it on, it uses one of your accessory slots, and it's tight around your shoulders, with some sort of odd rubber thing that just grips right down on them.
It's also really awkward, you think it'll hamper your ability to fight like this.
... You're also not sure how it'll function if you turn into a creature while using this.
Anyway, you go over the big gauge and the safety rules for how you can do this and not die. It seems that there's some magic in it so that you won't need to worry about depths, up until about two hundred feet where it'll just stop working all of a sudden. Depth is another big gauge... and the lake is more than 200 feet deep, but hopefully you won't have to go all the way to the bottom.
Filling it back up takes a while. You just leave it in the air and unscrew it and it'll fill itself automatically over the course of several hours.
I suppose there's nothing more to it other than to get going.
To be continued, unless there's more questions or other stuff to do.