-Katniss describing on how she won her Games in Catching Fire.
The 71st Hunger Games were won by the 17 year old Johanna Mason. These Games were quite popular, since the way she won them were very interesting. This year, Seneca Crane was Head Gamemaker. These Games took 9 days to complete.
Since she mentioned on how the tributes of District 7 wore costumes resembling trees for 40 years during Catching Fire, it can be safely assumed that she wore this outfit too.
Since she wanted to act weak and helpless during the Games, she sold her act by getting a 3 for her training score.
During the Games, as mentioned earlier, Johanna very convincingly acted weak and helpless, so that she would be ignored, but when there were only a handful of tributes left, she proved to be a vicious killer.