From the Super Mario Wiki
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Shuffle Shell is one of Mario's minigames in Super Mario 64 DS. While gameplay is based on Curling, the scoring is more reminiscent of Archery, and the name is derived from yet another sport, Shuffleboard.
In this minigame, the player must shoot Green Koopa Shells to the target found on the upper screen, using the blue paddle in the bottom of the touch screen with the Stylus. Every time the player shoots a shell to the target, he or she will score points. The amount of points will depend on the ring the shell has remained; the blue ring gives 100 points, the pale ring grants 200, the red ring 300, and the center grants 500. If any shell doesn't touch the target, there will be no points. The user can shoot up to five green shells to the target, but taking care with the other shells that has shot, because they will remain in the field. Knocking other shell will cause them to move around, changing considerably the points gained. Once the player has used all the shells in hand, the game ends showing results. Sometimes the target may change its location in the upper screen when a new game starts. Similar on using green shells on ice is Shell Smash found in the game as well.
Use the paddle to push the Koopa shells to the target. The closer the shells land to the center of the target, the more points you will earn.