SPST switchel will only let you short the middle point of the pickup (where both coils join) to either ground or volume pot/pickup selector, depending on the guitar. You short one coil, and you hear the other one.
So, you solder the middle/center conductor/s of the pickup to one lug of the switch, and the other one goes to ground or lug 3 of the volume pot, depending on which coil you want to hear.
This way, you simply short one coil when you pull/close the switch. Depending on how many conductors your pickup has, and which coil you want to mute, is exactly how you need to wire it.