I just use some blue tack to wedge the switch on a kettle down so that it continues to boil over (make sure you fill it so you don't risk boiling all the water off and cooking the element)
Cover any areas of wood that you don't want steam to get to with a combination of plastic bags and tape, then hold the dented area in the jet of steam for a few seconds, withdraw it and assess, repeat as necessary until you are happy.
As eluded to above, the steam strips the finish away completely, it looks and feels awful afterwards, so you have to be prepared to do some very fine grade sanding and finishing with wax or similar if you want a decent finish again. This is why it is important to cover up other areas of the bat.