I've watched loads of videos on YouTube on how to do pull offs but I can't seem to get the hang of it.

When I pull off/downwards as suggested I either bend the string slightly (but also noticeably) or I catch the string below. On all the videos the pull off sounds clean.

I'm obviously doing something wrong but I can't figure out what.

Would appreciate any pointers/tips that might help.

Put your index on fret 12 and your ring on fret 15 then pick the note on 15 (while keeping your index finger on 12) then while the note is ringing....flick your finger off of fret 15. Do it significantly hard. The trick is to Flick downward ...thus allowing the 12th fret note (E) to ring out.

Do it until you get it right. dont stop and go on to something else. do it until you got it.
mikesocarras is right on with the idea that you flick the finger instead of thinking of a pull. If you mute your strings and do the pull off motion you should hear a definite "pluck" sound, because what you're doing is literally plucking the string with your left hand finger. Another thing to watch out for that I see all the time in students that are new to the technique is pulling the string out of tune with the index finger. Say your index is on 5 and your ring finger is on 7, when you do the pull off if you hear the pitch changing make sure your index isn't moving sympathetically with your ring finger. The cure for that is finger independence, just go very slowly and carefully and concentrate hard on making sure only the ring finger or whatever finger you're pulling off with is the only one moving at all. Now for the problem of catching the string below, there are two techniques you can use. One is to move the finger down and out, so down just enough to flick off the string and then out, away from the fingerboard before it reaches the next string. You need this motion in classical pieces or in situations where you want to leave other strings open and ringing while you pull of. The second technique is good for rock or metal, or any situation where you have some distortion or you don't want any other strings ringing. In this instance you use the underside of the index finger to mute all the string below the one you're playing, so that when you pull off and bump into another string, you won't hear any extra notes because the string is muted. You'll hear a tiny extra percussive sound, but it will be very slight.
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