Open a crafting table
Place 1 paper in the crafting grid
Add 1 gunpowder to make 3 rockets
Add up to 2 more gunpowder for a longer flight duration
Use 1 paper and 1 to 3 gunpowder total
Take the fireworks rockets from the output slot
Use the rockets while holding an elytra to boost flight
Use the rockets in a crossbow to fire firework shots