Turn off the circuit breaker to your baseboard heater. Secure the door to your circuit panel so no one can turn the circuit back on while you work.
Remove your baseboard heater from the wall. The method will depend on the type of heater you have installed. You should be able to remove most models by unscrewing them from their mounting brackets or drill holes. Consult your heater's user guide if you are unsure.
Lay your underlayment and flooring up to the wall beneath your heater as per the manufacturer's instructions. Remember to leave an expansion gap between your floor boards and your wall. This will need to be filled with cork expansion strips.
Position your baseboard heater into place on top of your newly-laid floor and mark out new screw holes with a pen. The original holes will be too low now because your floor will be higher.
Drill new screw holes into your wall and refit your baseboard heater.