First you need a Miiverse account (it's easy and free). Then, when you want to screenshot something in the game you push the home button. That will pause the game. Then open Miiverse (it takes a while to do that on the 3DS) and Miiverse will say "Let's take a screenshot." ,or something like that. Then the shot will be saved to Miiverse. From there you can download it to your computer and upload it from there to TBT or anyplace else.