Car will not start, blinking immobilizer light

The immobilizer is preventing the engine from starting. Try arming and disarming the security system first. Ensure you have the correct key and use the spare one if you have it. Battery must be sufficiently charged as a weak battery would result in non starting and triggering the security system.

So how on earth do I do that?

Have the battery tested. Ensure it is good and sufficiently charged. Try a jump start.

Lock all doors and use the key to unlock or if you have a fob, use it to arm and disarm the security system.