think this is one of the last things to ask for or to expect - a step-by-step tutorial or instructional guides.
the community is generally a lot more helpful if you are asking specific questions, especially after you have tried building.
Anyway, i don’t have any instruction manual for you, what i can provide are the links to our multiple-balls catapults.
this is 8059a nbn 4-ball catapult (and also similar to what @ranOOm mentioned - 2 balls side-by-side).
this is 8059a this season 2-ball catapult (i normally call it vertical release of 2 balls).
do try to prototype a bit… and then ask questions when you encounter any issues 
PS… might sounds a bit like a publicity stunt - but you will really find many other approaches of launching balls (in the Blank. youtube channel) as well, eg. puncher, flywheel, etc
Edit: the initial link for nbn was not exactly the correct video (but it is good to see how the intake system, worked). have included the reveal video of the nbn robot.