Step 11: Fill the envelope
Your balloon should fill up nicely if you use enough reactants, but don't be surprised if it takes five or six hours. The reaction will start to die off exponentially, and all the little bits of aluminum will float to the surface because they've lost so much mass. At this point, you can leave it for another day to collect all the last bubbles, or just pop off the lid and be done with it.
During filling, use a wet finger to check the one-way valve where your tube enters the envelope for gas leaks -- If you find any, apply duct tape. I have this problem, as the valve on my balloon has a rip in it, so once the envelope is done filling I press a piece of duct tape firmly over the filling hole and that holds the gas nicely.
If you have a buffer bag, put a clamp on the end and let it collect the excess gas -- if it turns out you need more, you'll have some.
Here's a video of the envelope filling (I know there's no cooling basin, I added one later):