A tremolo is achieved with a fast smooth down-up-down-up-down-up-etc strum with the index finger. What you’re aiming for is the effect of a sustained, organ-like chord. Keep your hand relaxed and don’t overshoot the string area too much. You’re trying to get it to sound like just one fat string, rather than four separate strings. Another way of doing a tremolo, which you may find easier, is to lean the thumb edge of the hand forward slightly, towards the neck, and sort of wiggle the middle finger rapidly back and forth, grazing the strings with the left corner of the middle finger tip/nail. If you get a chance to watch a charango player, or a balalaika player (they use tremolos, too) in action, that will probably help.

You can practice any of these techniques on just one chord, in order to concentrate on the right hand, but adding chord changes or finger movement – using any spare left hand fingers to add any extra notes they can reach that sound good, using hammer-ons and pull-offs, etc – will make them all sound even better!

I hope you can follow my instructions – print is a lousy way to teach sound at the best of times! If it’s any consolation, I’ve sometimes come up with things I’ve really liked by completely misunderstanding what I was supposed to be doing, so have fun!

Good luck!