Instructions (Part I): Carefully view the below video for specifics on how to launch your 2L bottle rocket.
Instructions (Part II): Perform a series of test launches using differing amounts of water to determine the amount of water that yields the highest launch.
Instructions (Part III): Carefully view the below video beginning at the 2:25 mark. When done, decide if you would like to design a rocket built for maximum height, or maximum time spent in the air.
Instructions (Part IV): Using any combination of the materials shown below, build and test your rocket. When you are happy with its performance (max height or max time), make any corrections and prepare for final launch.