you can tap in anywhere and just splice into a wire required for it to start. For example, the small wire that goes to the little screw post on the starter. Not the big battery cable but the little one. That one sends the juice to the starter to let it know "go ahead and turn over". If you cut it, run a wire off each end to your switch. Basically, with this installed the starter would never turn over when the key is turned unless this switch was on.
You can also tap into a safety switch such as the one that requires the clutch to be pushed in to start. Tap into one side of it, split the wire and run them to your switch.