disconnect the front lambda from the harness and physically unbolt the rear from the cat
that is the quickest way as it is quite physically challenging wiring up the rear lambda and connecting to the underside of the car to stop the wiring from hanging down
the front is easy to hook back up through the engine compartment
worth unplugging your battery whilst doing this as your emissions will be different concerning your 02 readings, so the usual 50 miles under 3000 revs 🙂, i just take her for a cruise on the motorway as you dont kill your wallet on petrol stoppping and starting in traffic whilst the ecu is relearning, nice 65mph in 5th gear for the good part of an hour, use about a 5er of petrol
personally if u can afford a hundred quid then go buy a new cat without the pre cat as a lot of motor spares sell em as standard without precat, but that removes the element of knowing you did the mod yourself 😛