Most car alarm installers cut feeds at the ignition switch,then blend into the existing wiring.

Not sure what car you've got but I'm assuming its got an ecu etc then look at where the ecu is powered or earthed from and put your break in there,pull the plug to the ecu then solder in any connections,heat shrink them in properly so you don't have any future wiring issues with water ingress etc.
I say the key to any good immobiliser is making it look factory,not drawing attention with a big box with a toggle switch on it.

I'd break the wires for ecu,fuel starter and send them into a relay with 3 x normally closed contacts in a reasonably accessible location,then,I'd wire in an identical relay holder next to it,these wires don't need to do anything,put them under the dash etc then just remove the relay and swap to the dummy set of wires when you leave the car somewhere conspicuous,take about 5 seconds in total and baffle 90% of TWOCERS!!!!