Hello, my name is Mark and I will try help you with this.
Much depends on what type of system has been fitted and how well it was done.
What you will need to do is basically check the wiring to see what is power and ground and what is isolators for the immobiliser.
The power and ground are easy enough to take care of. You just need to disconnect them from wiring harness or fuse box depending on how they were fitted.
The difficult part is finding out what the immobiliser actually isolated. Some isolate the starter motor, the engine ecu, the fuel pump or a combination of these.
Normally the wiring would be cut and a section removed and the immobiliser wiring for that isolater would be attached to either end of the cut wire and a relay inside the immobiliser would reconnect the circuit when it was disabled.
You are going to have to work out what circuits the immobiliser was connected to, identify both ends of the wiring and reconnect them.
Hopefully it would have been fitted by an amateur and the wiring will just have been cut but no sections of it removed.