Follow the device design
Heaters differ in whether they must be fully submerged or can be mounted at a particular angle. Use the manufacturer's instructions and attach the guard when supplied.
A location near water movement can distribute heat more evenly, but avoid placing the heater where plants or hardscape block circulation.
Verify rather than trust the dial
Use a separate thermometer away from the heater to check the aquarium temperature. Compare readings at different locations if the tank is large.
Secure cords with a drip loop and inspect the heater during maintenance. Unplug it as directed before lowering the water level.
For your own tank
Place the thermometer across the aquarium from the heater and check for uneven temperatures. Keep the device away from dry operation and follow its minimum water-level guidance. Unplug it before a water change if the manufacturer instructs you to.
Use these as general hobby references. Test your aquarium and confirm product guidance before making changes.