From the Super Mario Wiki
Lakitu Launch is a minigame in Super Mario 64 DS and New Super Mario Bros.. It is one of Wario's minigames in Super Mario 64 DS and Action minigame in New Super Mario Bros. The player must use the stylus to slingshot slingshot in the giant Green Shells the Lakitus are carrying. At the end, the Spinies will jump out of the shells and will be counted.
In multiplayer, this game is similar to Bob-omb Squad. Only two people can play. The player must sling the balls and hit the Lakitus to earn points. If the Lakitu is holding a star, it should only be hit if it is on the lower screen. If it is on the upper screen, the other player will get points. Hitting Lakitus earns 100 points. Stars are worth 1,000. When the timer reaches zero, whoever has more points wins.
The multiplayer version of Lakitu Launch is Vs. Lakitu Launch. Lakitu Launch, the 1 on 1 minigame, is completely different than the single player version.
 In-game instructions
New Super Mario Bros.: Shoot as many Spinies as you can into the tubs within the allotted time.