Indeed, this quest is quite simple and quick for the XP & items rewarded and all three buildings are on a road so there are no monsters in addition to possible players. You do NOT however actually need to PK (player kill) if you are opposed to PvP to complete the tasks. You can simply go elsewhere if other opposing players are present. In fact, doing so is the smart move to complete this task quicker for each side.
You can only capture a structure if it is currently neutral or opposing controlled. Otherwise the status bar does not progress to capture.
As you enter a structure right click your portrait and under player-vs-player toggle 'Enable'. You must leave this setting on to complete each capture; toggling PvP immediately for 'Disable' to start your 5 minute countdown to PvE seems to stop the capture.
While inside each perimeter and flagged for PvP a status bar appears on your screen; Alliance blue to the left and Horde red opposite. A vertical white line appears and then an arrow to indicate progress. When the line reaches your faction symbol the building's flags change and your log updates. You do not have to capture all three structures in sequence or on the same day for the base quest.
As mentioned in other comments this quest is also repeatable from the quest giver for coin and xp. Each capture rewards a further mark which can be used to buy PvP style items such as rings and socketable gems from the same NPC. Most items run about 10 marks.
Each PK inside the structures also rewards a mark. However, be prepared for this fact as ruthless 70-80s who lack the courage to play on a full time PvP server occasionally gank much lower players given that excuse.
TheWoWDB
