how do you teach fleas to sing?

Fleas can be very difficult to work with, in my experience.

I'd suggest you use ticks, they handle criticism much better.

If you are dead set on using fleas, be sure to compliment them profusely, while offering suggestions as to how they can improve.

Expose them to the music of Queen, as they can always learn more from Freddie Mercury than you could ever teach them.

Use a metronome at first as well, until they get a good sense of rhythm without it.