Find a turtle on a beach biome
Feed two turtles seagrass
Wait for one turtle to lay an egg on the sand where it spawned
Protect the egg until it hatches
Raise the baby turtle with seagrass until it grows up
When it becomes an adult, it drops scutes
Collect 5 scutes
Craft a turtle helmet with 5 scutes and 1 helmet in a crafting table