Base level: get a good lock, lock to something sturdy in a visible location. Put a cable on the seat and through the front wheel. Register the serial number with police and hide an air tag on it.
Next level: cover your bike in stickers or hit it with spray paint in a few spots. Swap out components for kitchy ones or mark them up. Go wild with reflective paint on the tires.
Elite level… Hang out with bike punks. Ride with them and learn their ways. Even if your bike is stolen chances are it makes it’s way back to you.