Well its hard to explain my training based on John Myung's (Bassist of Dream Theater) video.

First you put your 1st finger on a fret and then hammer your 2nd finger repeatedly on the next fret


Then, you fret with your 2nd finger and hammer repeatedly with your 3rd finger


Lastly, use your 3rd finger to fret and hammer repeatedly with your 4th finger (This is tough in the beginning)