Step 1: Empty the water bottles and unscrew the screw caps.
Step 2: Drill two holes in the middle of the screw caps with a drill. It is reasonable to start with about 8mm. The holes may need to be widened a little later (depending on the type of bottle).
Step 3: Now the two lids are glued together. Hot glue or instant glue is well suited here. It is important that the glue point is waterproof.
Step 4: Actually, the preparation of the experiment is almost done. Now you only have to fill one of the two bottles with water to about two thirds. And then screw both bottles to the lids (careful so that the glue does not come loose).
Step 5, the experiment: (Well-intentioned advice in advance. If you want to avoid trouble with your parents, the best place to do this is outdoors or in the bathroom :-)). Now the full bottle is turned upwards. By shaking and rotating a little, a swirl is created by itself, through which the upper bottle empties into the lower bottle, as seen here in the film. It may take a little practice to ignite the swirl.