For the Worldcon at-con site selection is usually a department headed by the Site Selection Administrator with a half-dozen or so staff. Along with help supplied by each of the competing bid committees, this group runs site selection and counts the vote.
The site selection ballot is usually constructed so it consists of and 8.5x11 sheet with three sections. The top section is the ballot itself, the middle is voting receipt, and the bottom is a record of voting kept by site selection.
A voter (who is already necessarily a member of the administering convention) enters the site selection area, picks up a ballot and fills it out, possibly leaving the actual vote blank for the moment.
The next step is to buy a membership in the Worldcon being selected: WSFS requires that no one be allowed to vote for a convention they are not a member of. Payment is taken (and held in escrow for the winner) and the ballot is cut into three pieces, the receipt portion of the ballot is retained by the voter, the record of voting portion is retained by site selection, and the ballot itself filled out in not already done and deposited in a ballot box.
All of these steps are overseen by the site selection department and at least one representative of each bidder.
After close of voting on the final day, the ballots and the staff adjourn to site selection ballot counting.