In a round, every voice sings the same tune, but each starts a little later. Because the tune is written to fit with itself, the harmony happens by itself: you only need to know one tune and hold it while the others come in.
How to sing one
- Learn the tune until you can sing it without thinking.
- Sing it while one other voice starts a line behind you. Keep going, whatever you hear.
- Add a third voice, and a fourth.
The hard part is the same as in any harmony: keeping your place while another line plays. A round makes it easy, because what the others sing is something you already know.
Then sing every voice yourself
Record yourself singing the tune, then record it again starting a line later, and again. When you play them back together, you'll hear a round sung entirely by you.
Try it with a round everyone knows.