Gather the damaged trident, materials, and an anvil
Open the anvil interface
Place the damaged trident in the first slot
Place another trident or a Mending book if applicable in the second slot
Combine the items to repair the trident
Use Mending by gaining experience while holding or wearing the trident
Use Unbreaking to reduce future durability loss
Avoid using the trident until it is fully repaired if you want to preserve durability
Repeat repairs as needed with additional tridents or experience from Mending