There are two basic methods for straightforward rounds.
One is to write each "line" of the round on a separate staff, aligned vertically.
The other is to write the all the lines on one staff, and mark the places where each voice enters - often using an asterisk (*).
The title of the piece, or some other text, needs to make it clear that it is a round, and state the number of voices.
If the piece is more complicated (e.g. there are two different rounds going on at the same time, with different numbers of voices) it may be better just to write out the full score.
See https://en.wikipedia.org/wiki/Round_(music) for examples.