Step 35: Airflow Control Valve.
On the other end of the pen the Airflow Control Valve just goes in the end of the pen. In my case it had a lot of play so I needed something that would fill in the gaps. Luckily for me I had another ball point pen that fit snugly inside my first pen and the air flow valve fit snugly into it. So I just cut off a section and then glued and stuck them together. Depending on the pen you use and the air flow valve, you will have to figure out what will work for you. If nothing else, enough tape and glue can always fill the gap.
Also, on a side note, I cut off the shirt clip the pen had on it to get it out of my way using the Dremel tool.