bubbles, its on the instruction book - I used gonal-f, stick the needle on the pen, dial the end to 37.5 (I don't think I pulled the end bit out), take the safety caps off the needle, hold the needle to face the ceiling (vertical), then flick/tap the pen a few times with your index finger, you will see a fine drop on the tip of the needle - then the pen is primed - you just need to get the airbubbles out, so as soon as the drop appears, it means the needle is full of fluid.

have fun and don't forget to aim for the fattie part