You should be able to program the remote (key fob) through your DIC (driver information center) if the option is available. When you start programming the horn will honk & then you press the lock & unlock buttons on the remote being programmed. When the remote is programmed the locks should cycle or the horn will honk, then you will need to repeat the process for each key fob after that up to a max of 4 remotes. If your veh does not have this option on the DIC you will need to take it to a dealership to have it programmed.
Source: GM Certified Technician