Place 4 End Crystals on the 4 bedrock pillars at the End (one per pillar).
Ensure each End Crystal is sitting on top of its corresponding bedrock pillar.
Create the End Crystal by placing a Ghast Tear in a crafting grid with 7 Glass blocks (and then place the crystal on the end pillar).
Obtain the required materials:
Ghast Tear (from a Ghast)
Glass blocks (smelt sand)
End Crystals require additional placement via the crystal block item.
If the dragon is still alive, kill it first.
After the last End Crystal is placed, wait for the respawn to occur.