Every time you spawn on the boat (i.e. after a dive or a time skip) check in with your GYAO.
- If it sleeps: Turn off light
- If it's sick and doesn't sleep: heal it
- If it doesn't sleep, you can feed it, to reduce its hunger and increase it's love. This is optional I think.
Different amounts of hearts and hunger will result in different GYAO at the end, but it doesn't seem to matter. If you raise the same one 5 times, you will also get the achievement.
- Once the GYAO developed into the fifth stage (i.e once it's not just a generic worm/fish) you can let it die. Just ignore it for a day or two and it will die.
- Once it's dead, revive it and start the cycle again.
Tips:
- If you're in the end game and have nothing else to unlock, just use a time skip. It saves a lot of time.
- You can cancel the animations when interacting with the GYAO by pressing ESC. This saves you some time as well.