Depends on whatya wanna do, how elaborate you want to make it.
Simplest route is a hidden switch, maybe one like
this one from Speedway Motors that can cut the battery & ignition. Mount up under the dash wehre it's hard to see or in a fender, so long as your hood locks it's pretty hard to bypass.
I've seen simple circuts that use relays powered by a hidden switch that cut the battery and ignition circuts, or a variation on the theme that require you to turn on the key then pass a magnet over a hidden reed switch to engage the relay, shutting off the key resets the whole thing.
