A firework rocket is paper and gunpowder. One gunpowder is a short boost. Two or three last longer, up to the crafting limit. That plain rocket is what you want in the air.
The safe flight rocket
- Craft paper from sugar cane.
- Collect gunpowder from creepers, ghasts, or a witch farm you trust.
- Put paper and one to three gunpowder in the crafting grid. No firework star.
- Wear an elytra, jump, and press jump again to glide.
- Use the rocket while gliding. It boosts you in the direction you are facing.
Stars are for shows
A rocket with a firework star bursts. On Java that burst can hurt you if you are the thing it hits. Keep starred rockets for the ground show and plain rockets for flying.
Elytra come from end ships after you have been to the End. Rockets are the fuel. They do not replace the elytra, and they do nothing useful if you are standing still on the ground except make a noise.