On the first floor of cellblock A (the one you start in), more or less directly beneath your cell is a locked storage room. There are a few different ways to get in here, including finding the code (which I think is inside a stuffed rabbit in the showers) or by trading for a multitool. Make sure the guards aren't nearby nor the camera pointing your way when you open the door or enter the off-limits storage area. Grab the basketball from inside here, take it to the yard, and toss it through a hoop to unlock the achievement.