Very very easy.

Index finger = 1st finger.
Middle finger = 2nd.
Ring Finger = 3rd.
Pinky = 4th.



This is just an example:

With your SECOND FINGER, press down on the B string's 3rd fret (can be any string or fret). Strum the string. With your THIRD FINGER (while keeping the B string pressed down with your second finger) bring it down fast onto the 4th fret.

/hammeron.


To do a pulloff, just quickly lift off your THIRD FINGER.




NOTE:

During my example (assuming you do the pulloff I suggested right after the hammeron I suggested) your SECOND FINGER on the 3rd fret DOES NOT move.


Hope I helped.