Ensure the End is entered with the Dragon alive (not already defeated)
Defeat the Ender Dragon by killing it while it is on the main island
Wait for the respawn to trigger by killing all Ender Crystals on the outer obsidian towers
If the dragon is already defeated and you want it to respawn, place 1 End Crystal on each of the 4 End portal bedrock blocks on the outer towers
Use End Crystals that can be crafted or obtained, then place them on the bedrock blocks atop the obsidian towers
After all required End Crystals are placed, wait for the respawn sequence to complete
The Ender Dragon will spawn and the fight begins