Step 6: Carbonation
You've got three weeks until your cider is done, this should be your in between project. They are very handy, you can make your own sodas, you can re-carbonate flat soda, they really do pay for themselves.
I bought a 20lb CO2 tank at a swap meet for $10 and it had a regulator. I traded that tank in for a new full tank for another $30. I bought a cheap air compressor kit from Harbor Freight
Another thing you will need is a carbonation cap that fits a 2 liter bottle.
Here is a good video of how to make a carbonation cap.