Fill the bottle with water and then put the cap on, or put the cap on first and then fill it with water, similar to filling a glass bottle. Ensure the caps are not forgotten; they are generated in dungeon chests.
You can throw or place the rocket down and Sneak+Right Click to activate it. Upon activation, it will cause knockback and deal damage if it hits something. When it hits a solid block in Survival Mode, it will drop 4 Plastic Scrap and 6 Paper, allowing you to recover some materials. However, be aware that it\'s not highly accurate and may deviate significantly.