a scan tool is absolutely required for this and is done as much for your protection. Otherwise anyone could purchase a key fob off Ebay and self-program to use on your vehicle. If you have heard that it can be done without a scan tool, that source is incorrect.
Here is programming method per GM:
Attachments are only available to registered users.
TRANSMITTER PROGRAMMING IMPORTANT: All transmitters which are to be recognized by the remote control door lock receiver (RCDLR) must be programmed in a single programming sequence. If the system is placed in program mode, all previously programmed transmitters will be erased upon the receipt of the programming signal from the first transmitter. Up to four transmitters may be programmed. The order in which the transmitters are programmed is important. The first transmitter programmed will be transmitter #1, and the second transmitter programmed will be transmitter #2. The number on the transmitters are for reference only: #2 can be programmed as #1, or vice versa. Additional unnumbered transmitters are also available. Use care to program the transmitters correctly.
IMPORTANT: The scan tool must stay connected until all of the transmitters are programmed.