with the Key in the "On" position.
You need to find a power source so you can run powr to the new switch.
You need to find the wire that goes from the keyed switch to the starter,
Cut that wire and wire it into 1 side of your switch, power goes to the other side of the switch. I would suggest putting a fuse in line.
Cole Hearse makes pretty good stuff.
you should be able to turn the key to the "on" position and then press the starter button
Good luck