Haven't done it in a while since I'm on a pump at this point so bare with me.
Have you seen a a penfill refill? There is a rubber stopper on one end where the plunger in the novopen pushes against and on the other end is the rubber membrane where the needle enters the cartridge. On the end where the needle goes look at the collar that locks it in the pen. You can pop one off to keep and use with cartridges you pull out of disposable pens. You need to have one to keep the cartridge you remove from your disposable pens seated properly in the reusable pen.
Look at your disposal pen, you will see that there is a glass cartridge inside the plastic pen. You can carefully shave the plastic barrel with a serrated steak knife to remove the glass cartridge. Do this midway on the clear grid. Once you have the cartridge out, if you do not have a collar from a penfill (they will snap off) to put on it, then very carefully cut off the front end of the disposable pen and trim it so it seats properly and locks your cartridge in the novopen. You may have to experiment a bit on how much of the end you need to keep. The threads are where your needle screws onto so need it for the pen to work. Also be aware that the label is on the plastic not the glass cartridge, so if you use more than one kind insulin that you need to label the cartridge or the pen so you know what you are using. No fun injecting rapid when you think your using long acting or vis versa!
I was able to use both long acting as well as rapid insulin in the pen juniors.
I just got to the point that I needed a smaller dose, so went to pump, and then to pumping U50 insulin. Will be pumping U10 soon as my activity increases with the good weather.
Be aware that the pens will do half units but only with a base dose of at least 1 Unit.