*These detailed procedures are only used if there are 3 or more candidates on the ballot.
a. Each ballot is counted for first preference votes. If a candidate receives more than 50% of first preferences, they are elected and the process is complete.
b. If no candidate receives more than 50% of first preference votes, then the candidate with the fewest first preference votes is eliminated. The second preferences (or next preferred candidate that hasn't been eliminated) from the ballots cast for the eliminated candidate are then added to the remaining candidates. This process is repeated until one candidate achieves more than 50% of the votes, and is declared elected.
c. Should there be a tie for the fewest number of first preference votes, then among these, the candidate with the fewest number of second preference votes shall be eliminated. Should a tie persist, the procedure shall continue with third preference votes, etc. Should a tie still persist, the candidate with the fewest number of original first preference votes, shall be eliminated. Should a tie still persist, the procedures shall continue with original second preference votes etc. Should a tie still persist, a candidate shall be eliminated by random draw. Multiple candidates are never eliminated simultaneously.
d. Should there be a tie between the two top candidates (and thus no candidate receiving more than 50% of the vote) a new election shall be held.
See short YouTube clip here explaining the mechanics of Instant Runoff Voting (know as Alternative Vote in the UK).