To tag a reservation we must first create a reservation tag. Now we have the option of tagging reservations when we create them or anytime after.
From the reservation detail page, we can tag the reservation by using the tag drop-down menu.
Now we can see the tag on the reservation from the detail page and from the occupancy map we can see the tag icon.
By selecting the reservation we can see what the reservation is tagged with as well as add or remove tags.
Here's a video offering a walk-through: