You and a partner build a contraption to move balls along the Great Ball Contraption
Check out some example builds, and then either make a copy of one you like, or start to design your own from ideas you get from these examples.
Beyond the Brick
JK Brickworks Playlists
Example Modules
Nico71
JK Brickworks
ReBrickable
Other resources
LDraw - CAD for LEGO. Also allows making Build Instructions