How it works
List the teams, pair each one against every other, and spread the games over rounds so no team plays twice in the same round. With 6 teams that is 15 games in 5 rounds. After the last game, rank teams by wins.
How many games?
Each of n teams plays n − 1 games, and every game involves two teams, so the total is n × (n − 1) ÷ 2. Four teams play 6 games, eight teams play 28, twelve teams play 66. The number grows fast, which is why big fields use pools or a bracket instead.
Odd numbers of teams
With an odd number of teams, one team sits out every round. A 5-team round robin has 5 rounds and 10 games, and each team rests once. Use the rest round for refereeing or scoring.
Who wins
Teams are ranked by wins (a draw is usually worth half a win, or one point against three for a win in soccer). Ties go to the result between the tied teams, then point difference, then points scored. Decide the order before the first game and write it on the sheet.
Single and double round robin
A single round robin plays each pairing once. A double round robin plays each pairing twice, once on each side, which is how most football (soccer) leagues run and a fairer test when home advantage matters.
Questions
Is a round robin the same as a league?
A league is a round robin played over weeks, often twice. The format is the same; the calendar is longer.
When is a round robin a bad idea?
When there are more than about 10 teams and only one day. Ten teams need 45 games.
Does the order of games matter?
Only for the last round. If you can, play the games that decide the standings at the same time, so no team knows what it needs.