How to do the golf balls in a locker prank?

Ok, so ive seen youtube videos of people putting golf balls in someones locker, and they open it and they all fall out, and im planning on doing this.

But, how do they get the gofl balls in there without having them dump out while they put them in there?

Thanks
I am in middle school, its the long lockers so i would need a big piece of card board.

Also theres cameras…so it would be kinda hard to do it without being caught but, ima try to do it.