Take 2 or 3 rubber bands, depends on the thickness. The rubber bands go through the hole on the propeller and goes inside the boat and then out on the front (see the second picture).

The clearance is small for the propeller, if you have problem with elephant foot then probably you have to widen the hole a bit.

There is no bearing here so if you have a small rubber band it will pull the propeller forward too much and the friction will win. Try to find a rubber band approximately the length of the boat.

The propulsion is not great, but the boat swims well, I don't want to make it more complicated. My 2 years old daughter is very happy and that's the only thing that matters to me.