Step 12: Final Preparations...
At this point, you should have a full envelope and a completed blimp frame. Find the frame's center of mass -- it should be just behind the battery -- and line that up with the central crease in the envelope ( it comes nicely folded). Now lower your envelope until it's touching the four pylons; anchor it and make sure it's symmetrical around the long axis. Take four velcro patches and press them firmly onto the envelope, directly above the pylons, so that you can sort of pinch the velcro on the mylar onto the corresponding patch on the pylon. Do this for each pylon, and the frame should be securely fixed to the full envelope.
Now, the moment of truth. Release the anchors, keeping one hand on a string at all times. If you're lucky and you kept the weight down (my frame is just under 130g, you can get by with 140g), the whole blimp will slowly rise up. If it does, congratulations! Add some payload to get to your desired buoyancy and take to the skies.
If your blimp fails to rise, do not give up hope. It took me six months to get my blimp fully airborne, you shouldn't feel bad if you don't get it right the first time. Weigh your frame, and do whatever you have to get it 130 grams. If that doesn't do it, your gas is contaminated with air, and you need a fresh run with a better seal between your tube and envelope. Make sure there is no gas leaking from your reaction vessel or tubing.
On an unrelated note, my envelope as seen below is speckled and partially translucent because I spilled some lye solution on it, not because of some error committed by the envelope manufacturer. I'm an idiot. Don't be an idiot.