Breaking the Ice
Start the game with players shooting single free throws in succession until one player makes a shot, which breaks the ice and counts as 1 point. The scorer shoots from the line again until she makes two more, or misses, with a missed shot available to be rebounded. If she makes three, she "checks" the ball at the top of the key and tries to score a conventionally contested basket against all the other players, with live play until someone scores a basket.
Live Ball Off the Rim
A player rebounding a miss must clear the ball by dribbling outside the 3-point line before trying to score himself. All other participants play cutthroat defense, all against one, trying to prevent him from scoring. A live ball score gives whoever made the shot 2 points and puts him on the free-throw line for a chance at more. Shoot for 1 point each until you miss or make three, with rebounds live for all players. As before, a player who makes three in a row checks the ball and tries to score against the defense while they each try to steal the ball, block a shot or rebound the ball, and score themselves. The ball is live until someone scores against the mob-style defense, putting them on the line where the cycle continues.
The Rules of Play
All basketball rules apply except for out-of-bounds. If a player is fouled, she gets the ball back with a check at the top of the key. If she travels or double-dribbles she must intentionally miss a shot from the free-throw line to give up possession. Live ball scores count as 2 points and the resulting free throws count as 1. You may choose to include behind-the-arc 3-point shots, not just the conventional 2. Each player keeps an individual score.
Winning the Game
A player wins by scoring exactly 21 points. Go over 21 or miss a free throw with a score of 20, you bust and your score rolls back to 11, which adds the drama of strategically missed shots. Optional score-reducing rules include "tips," in which a defender can tip a rebound back into the hoop without clearing the ball; and wiping the score of the shooter who missed back to zero or 11, whichever is closer. And most versions of the game force the player who scored 21 to shoot a 3-pointer. Make it and win. Miss it, the score rolls back to 11 and the game continues.
Rules can vary wildly. High-level players may choose to shoot 3-pointers instead of free throws. Some regions reduce scores to zero if you end a round of free throws on 11. Sometimes points roll over from one game to the next, allowing players who scored less than 11 to keep their tally. The point of 21 is to have fun. Optional rules aim to level the playing field so that everyone gets involved, and the game is intentionally difficult to win so the fun lasts. Don't be afraid to invent your own version of 21 that's right for you.