The spot welder is a modified microwave oven transformer.
There is a momentary switch wired in series with the power switch on the primary (120v ac) side of the transformer (so be mindful of the pushbutton switch. it has tape over the contacts, but be aware there is 120v ac in there)
with the main switch off, position the pieces you want to spot weld. The resistance needs to be low, so you might need to sand off oxide layers. Make sure everything is aligned (which can be tough since the welder isn't very rigid), then turn on the main switch, and press and hold the momentary switch for a second or two.