A pull-off is pretty well using a fretting hand finger to pluck the string. Whatever you may do to make a string sound, using a r.h. finger, the same will apply to produce a pull-off. You must pluck the string in an upwards or downwards stroke, at rightangles to the string, with your r.h. finger. So, the same applies for a pull-off. If you're pulling off to another fretted note on the same string, press a little harder with the second note finger, in order to stop the string being pulled sideways and out of tune.
A good practice regime is to play, say, top string fret5, hammer on to fret 6, pull-off to 5 again. Keep this going until each note is the same tone and volume, then change fingers - for both notes when it's good- and all without stopping a regular rhythm. Slowly at first, of course, and don't rush either the hammer on or the pull-off.