yay i can play tht song so i can b of some help
but hopefully u will understand me
well you just need to keep at it really(practise)
there are two different ways to do a pull off
1:like in the case of this song by nirvana you have to strike the 2nd fret and using that momentum you have to hammer on the 3rd fret and release quicly
2:in some other cases when there is just a pull off first and no hammer ons.you have to strike that note and let go to the other note
EG: e(string)------3p2--
there u strike the 3rd fret and release to the second fret..so ...ur fingers should b holding both frets at the start and then when u strike the first note take away the finger on 3rd fret and then u have accomplished a pull off!(as simple as that)
hammer ons are actually harder than pull offs as u need to hit the string accurately and b careful not to hit onther strings at the same time,pull offs require no such strain
a good excercise is
e --1h3p1------(repeated)
to do this excersie you have to just hit the string ONCE and then use your momentum to hit the 3rd fret and release it quickly so it doesnt lose pitch.just keep your finger stationary on the 1st fret u dun have to move it in this excercise