How to Play Hammer-on Notes on Your Ukulele
Pluck the string as usual.
Leave the string ringing while you quickly bring a finger down on the string (or back down on the string if it’s fretted).
Try to make most of the hammering movement come from the knuckle — not your wrist — and land in the usual fretting position just behind the fret wire.
The word hammer is very apt because you have to bring your finger down quickly and firmly for the technique to work. If you hammer down hard and cleanly enough, you should hear the string still ringing without you having to re-pick it.
In tab, a hammer-on is shown as a tie between the notes with an h above:
You can also hammer-on whole chords! You strum the open strings and then hammer-on all the notes of the chords. The tab for a sequence of hammering-on the first chord and strumming the rest of the bar looks like this:
Alternatively, you can start with one or more of the strings fretted and then hammer-on the remaining notes in the chord: a partial-chord hammer-on. The tab here shows a progression based on F and A with hammer-ons (and is a technique used often by Zach Condon of the band Beirut).
You can hear and play along with fretted, whole-chord, and partial-chord hammer-ons in the audio clip.