Tremolo is the picking of one note in rapid secession. You can use the nail on ether your thumb or your index finger (or any other finger that works) to produce tremolo.

On sheet music tremolo looks like this:

To create tremolo with your index finger or thumb, first anchor any number of your right hand fingers to the soundboard below the sound hole. Use these fingers as a pivot point. Place your thumb(nail) or index finger on the string (you can brace your thumb with your index finger and vice-versa) and then pick down. Do this without bending your thumb or index finger – instead rock your whole hand forward on the “pivot fingers”. Picking up is just opposite. Practice picking down and up consistently, speeding up, slowing down, changing strings.

A more advanced way to produce tremolo is with a roll of your fingers. Start by picking up with your middle finger,and then with your index. Follow those two with a down stroke of your thumb. This technique is hard to do on any string but the bottom. Start slowly and as you get the hang of it speed up. Bryan Tolentino uses this kind of tremolo in “G minor fleas” that he did with Herb Ohta Jr.