Craft or obtain an Elytra
Equip the Elytra in your chest armor slot
Obtain Fireworks Rockets
Make sure the Fireworks Rockets do not contain a Firework Star if you want safe flight boosting
Jump from a high place or start gliding in midair
Press the use item button while gliding with the Fireworks Rocket selected
Keep using Fireworks Rockets to gain speed and stay airborne longer
Aim slightly upward to gain altitude and distance
Avoid using rockets with explosions if you want to minimize damage risk
Carry extra rockets for long-distance travel