The Hugo final ballot is typically released in late April and lists the nominations (the short list) for each Hugo category. (This short list was arrived at by means of the nominating ballot process.) The voting deadline is typically about four weeks before the Worldcon.
Unlike on the nominating ballot, voters on the final ballot are limited to members of the current Worldcon. Members who join the current Worldcon after the final ballot is released are sent their voting information.
Most balloting is electronic, though paper ballots are always distributed and accepted.
Voting for the Hugos is by transferable preferential ballot, often incorrectly called the Australian Ballot. In each Hugo category, the voter is invited to rank the nominees and No Award from 1 to 6 (or more if there were ties resulting in more than five nominees.) The transferable preferential ballot is constructed so that it is always to your advantage to rank all nominees you feel qualified to vote on and who you feel deserve a Hugo in the order in which you would prefer to see them win.
Once the voting period is over, the Hugo Administrator counts the ballots using the transferable preferential ballot, confirms that more voters prefer the winner than prefer No Award, and creates a complete breakdown of the voting for the Hugo voting report.
And then keeps the result a deep, dark secret until the Hugo Ceremony.