DH’s explanation holds for the Carling H 494 (SPDT) switch. BUT, you really need a multimeter. To confirm continuity between connections, check grounds, and measure voltages, you need a meter. Included in the minimum tools you need to build an amp are a multimeter and a circuit limiter (light bulb limiter). Google “light bulb limiter circuit” or use this link:

Without a current limiter, you run the risk of frying the PT (and/or other components) when you fire up your amp for the first time if there is a dead short somewhere and you need the meter to confirm the grounds are correct. Speaking from experience, I would advise you to get a meter and build a circuit limiter before continuing with your amp construction.