To do this, go to your friend feed, and view a milestone whilst on the TA app.
Easy milestones to view are completed games, TA score milestones, gamerscore milestones, games played milestones, etc.
Edit #1: It doesn't even have to be your milestone - it can be the milestone of anyone on your friend feed.
Edit #2: Make sure that you have your friend feed set (on the site, NOT on the TA app) to actually SHOW milestones. If you do not, this will stop your ability to unlock this achievement.