start very slow. think of each movement of a hammer on in two parts. 1 being the pick hand and 2 being the fret hand. 1-2 1-2 1-2 1-2. start off counting something like this 1..2..1..2..1..2..1..2..1..2 and work your way up to 12.12.12.12.12.12.12. starting slow and building speed is the best way to ensure that you're concentrating on the proper movements and getting the most accuracy.

try practicing with your stronger fingers first to get the right sound... then work on your weaker fingers (pinky and ring finger) to gain the strenght and speed.