You can only get this achievement while you are also unlocking a Challenge. So both of them will be unlocked together.
Open the TA app, and look for a game that has challenges that you can unlock (I would suggest going to "My games and apps" and selecting a game from there). Once you are on the game page, hit
and you should be on "Rate game." Hit
until you get to "View x challenges" (where x is the number of challenges available in that game). Hit
on that and you will have a list of challenges. An open book sign or a rolling camera sign will tell you if there are solutions for that challenge. If there are no solutions for it, but you know it is easy and you can get it, go write a solution for it (yes, your own solution counts). If there are already solutions for it, hit
on it and then hit
and you will have highlighted "View x solution" (where x is the number of solutions available for that challenge). Press
on that and you will see the solution for that challenge.
Once you are viewing that, either double tap
to "Snap an app" and then press
and you will have TA snapped. Or go to your home, and choose "Snap" and choose TA from the list there. Or, if you have a Kinect, simply say, "Xbox; Snap Trueachievements"
Now that it is snapped, simply play your game, and when you unlock your Challenge, you will also get the TA achievement at the exact same time.