Adding footswitch to 2 channel amp

[kin0]

Okay I have two channel amp which uses 3pdt switch to change between channels (input,output,power). How can I add a foot switch which will work with a pl jack?
I read this

if the switch is a DPDT latching Type (like most small solidstate practice amps) then all you will have to do is use a DPDT relay and solder it to the same terminal as the original switch on the PCB (you can even leave the original switch intact) now run the trigger wires to a 1/4" input jack & "steal" the required voltage (usually 12VDC) from a suitable location on the PCB and she'll be even bigger apples

but I don't understand anything from it.