Milestones can be marked as such within the Task Information box. Or, perhaps an easier method is to set a zero (0) duration on any task

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you'd like to designate as a milestone.
To create a milestone in Project 2007 or 2010, follow these steps:
- Type a new task into the Task List. You can make this entry in any view that has the Task List.
- Some people like to give it a descriptive name. Whether or not you do so, Project will identify it as a checkpoint, or milestone.
- Set the duration of the task to zero days in the Duration column.
Note: If you are accessing the Task Information Box, you can do so through any view that shows you the list of tasks. All you must do is double-click the task and the Task Information Box will open. You can also select the Task tab and look in the Properties group; you'll see the Notes icon there and you can simply select the Advanced tab.
The image to the right is a simple list of tasks relating to a local high school band--their band nights, games, concerts, and more are listed. Initially I just entered the games and dates. However, I wanted to set Cleveland John Adams as a milestone because it is the first actual football game of the season, so I tabbed over to the Duration column and entered a zero. I have opened the Task Information Box and checked off Mark Task as Milestone. Either method works equally well.
Milestones are displayed visually on the Gantt Chart as diamonds. This makes milestones easily recognizable as a project checkpoint and easy to locate in any Gantt Chart.