Then open the valve of your launch pad and water will flow into the rocket. Make sure the corrugations run along the 6 inch length of each fin, for strength. Fill the bottle one quarter full of water and push the cork in tightly. To use your filling system, simply fill water into the pressure sprayer and build up pressure using the integrated pump. Each fin is 4 inches wide and 6 inches long. How to make a Bottle Rocket Decorate the bottle with the cone and fins. It is important that you fill the water into the rocket before you build up … Take the bottle outside and connect the pump to the needle adaptor. Use the pen to mark the outline of the three fins on the coroplast or corrugated cardboard. Making the Homemade Water Rocket.