How a round works
In round 1, split the field in half and pair the top half against the bottom half: with 16 players, seed 1 plays seed 9, seed 2 plays seed 10, and so on. After each round, give a point for a win and half a point for a draw. Then pair players on the same score against each other, and never repeat an opponent.
How many rounds
Enough rounds to separate the field: about log base 2 of the number of players, rounded up, with a minimum of three. That is 4 rounds for 16 players, 5 for 32 and 6 for 64. More rounds make the top finish clearer, but the first few rounds do most of the work.
Odd numbers
If there is an odd number of players, one gets a bye each round and scores a point for it. Give it to the lowest-ranked player who has not had one yet, and never give the same player two.
Breaking ties
Players will tie on points. The usual first tiebreak is Buchholz: add up the scores of everyone you played. A higher total means you faced a tougher field. Chess adds others, such as Sonneborn-Berger and the result between the tied players; a playoff game settles a tie that matters for a prize.
Questions
Does everyone play the same number of games?
Yes, apart from byes. That is the point of Swiss.
Is Swiss good for a one-day event?
Yes for 8 to 64 players, if you can pair quickly between rounds. This site pairs each round for you.