Place on your vaporizer so that the max setting is 2 o'clock for VB1.0 or 3 o'clock for VB1.5. First turn the shaft all the way clockwise and then place the knob on at 2 or 3 o'clock position depending on your model.
Don't know what model you have? VB1.0 is the classic and 1.5 has visible glass beads inside the heater. VB1.5 has only been made in the last few years.
Splay the metal shaft with a flat screwdriver if you want the knob to be tighter, or use superglue to keep it on.