The shape
Twelve teams might be split into three pools of four. Each team plays the other three in its pool, which is 6 games a pool and 18 games in all. The top two from each pool, plus the two best third-place teams, make an 8-team single-elimination bracket.
Splitting the pools
Seed the teams first, then deal them out in a snake: pool A gets seed 1, pool B seed 2, pool C seed 3, then pool C seed 4, pool B seed 5, pool A seed 6, and so on. Each pool ends up with a similar mix of strong and weak teams.
Ranking inside a pool
Rank by wins. For ties, use the result between the tied teams, then point difference, then points scored. Some events cap the point difference per game (commonly at 5 to 8 points) so no team runs up the score.
Wildcards
When the number of advancing teams does not fit a bracket, add wildcards: the best remaining teams across pools, ranked by winning percentage then points per game. Compare per game, not in total, if pools are different sizes.
Seeding the bracket
Do not let pool-mates meet in round 1 of the bracket. Seed the pool winners first, then the runners-up, and pair each pool winner with a runner-up from a different pool. The brackets here are built that way and tested for it.
Questions
How big should a pool be?
Three to five teams. Pools of three are quick but one loss can end hopes; pools of five are fair but take many games.
Pool play or straight elimination?
If travelling teams expect more than one game, use pool play. If time is tight, use single elimination.