Cut of the top of the bottle. You can just throw the cap away but keep the top because you will need it later.
Take the top and put it back on the bottle but reversed, like on the picture below.
Now as to how it works.. When a fish enters the bottle and wants to get out it will follow the natural curves of the bottle. That's why goldfish in a round aquarium swim in circles alongside the walls of the aquarium. In a normal bottle the fish would have no problem escaping but this bottle trap, when the fish follows the sides then the bottle will lead the fish back inside... it can't find it's way out.
Next time you want fish on your plate remember this trap, it's easy and cheap and very effective.