There are two possibilities here and it will all depend on whether your car is a 2005 model (that was first sold in 2006) or a post-facelift model and also whether it’s a TS Astra or an AH model Astra. If it’s the earlier car, then you might be in luck. The process involves taking the new key, placing it in the ignition and turning the ignition on (without starting the engine). Then, when all the dashboard lights are lit up, you press one of the remote buttons on the key. And that’s about it. You should hear the car lock and then unlock itself to let you know the process is complete and you’re good to go. For post-2005 models, however, things are not so simple. In this case, you need to either visit a locksmith or a workshop that has the Holden diagnostic gear, as the new key needs to be coded to the car via the diagnostic port. That said, I’d give the first method a try before spending money at a workshop. Who knows, you might just get lucky.