A player can choose to Visit a Port at any time during their turn if their ship is located on a hex where there is a Port, marked by a banner with a printed name on the Map Tiles. Players can visit more than one Port during their turn, but they can't visit the same Port twice in the same turn.
Visiting a Port is done by declaring that you do so, Banishing any enemies that may be in your On Deck area, and then turn up the top card of the Denizen deck. If the Denizen card is a Passenger whose destination is the same Port that you're visiting, discard the Denizen card and draw a new one. The Denizen card makes an extra action available, in addition to the actions that are available on the Port Board.
The player may now freely move Crew members, one at a time, from their Ship Board to any location on the Port Board. Moving Crew in this way doesn't require spending any Crew Order tokens, but once a Crew member has been placed on a Service location (marked by a circle) on the Port Board, it can't move again until you leave the Port, unless some special ability makes it possible.
Placing Crew members in locations on the Port Board unlocks the Service action(s) associated with that space. At a few locations, the Service action is triggered without extra cost, but for most locations, unlocking the action doesn't trigger it; you need to pay a cost to trigger the action. Most Services on the Port Board can only be triggered once per visit, but there are three exceptions: Install, Sell and Stay A While And Listen, which are marked with a recycling arrows icon to indicate that they can be used multiple times. Refer to the Port Board for further details about the actions.
If you hire crew at the Port, the newly hired Crew members go to spaces in your Below Deck area. The newly hired Crew members can then move to spaces on the Port Board and unlock Services. (Not mentioned in the rulebook, but confirmed by game designer.)
Once you have finished visiting the Port, move your Crew members back to any valid locations Below Deck on your ship. If you Crew members are not Fatigued, they can trigger Crew effects by moving in this way. You also get to turn your ship around to face any direction without any extra cost when leaving a Port.