The surprising thing about this design is that the toy floats really high on top of the water, since the can weighs next to nothing. The upward force making it float is equal to the weight of the water it displaces when submerged, so it's fun to use. Heck, you might even want to make a raft or floating chair out of this stuff.
The picture explains it all. Just remove the bottom from a few cans, then stack them on top of a normal can. The final can has only its top removed before being shoved on the pile. The tricks here are (i) to be careful in deburring/smoothing out the open rim, (ii) to apply smooth, steadily-increasing, and steadily-building pressure when inserting the cans into each other, and (iii) to poke airholes in the outer can that you'll block later. If desired, you can tape the seams together with packing tape, duct tape, or an equivalent watertight tape for that perfect airtight seal.