Hi @needlesandmath,
Here is the original post on refilling the pen. It is from 9 years ago!
refilling-the-lilly-kwikpen-disposable-pen
Just pay attention to the direction of the pushing and turning, etc.
Inevitably get a small bit of air in there. The small amount of air is not a problem because if you are holding the pen downward when injecting, the air will be up at the plunger not the needle. Also, you can suck the air bubbles out with a syringe if you want.