[ar0ck]

I have this saved for my 2008 Silverado:

Turn "ON" the ignition, with the engine OFF.
Apply the parking brake. IMPORTANT: If the learn mode cannot be enabled, ensure the TPM system is enabled in the PDM.
Turn the exterior lamp switch from "OFF" to parking lamps 4 times within 4 seconds. A horn chirp will sound and the low tire pressure light will begin to flash indicating the learn mode has been enabled.

OR

Holding Lock & Unlock buttons on the Key Fob

IMPORTANT: If a horn chirp does not sound after 35 seconds for any of the 4 sensors, turn the ignition "OFF" to exit the learn mode and repeat the procedure from step 1.
Starting with the left front tire, increase or decrease the pressure for 8-10 seconds, or until a horn chirp sounds. The horn chirp may occur before the 8-10 second pressure increase/decrease time period has been reached, or up to 30 seconds after the 8-10 second pressure increase/decrease time period has been reached.
After the horn chirp has sounded, proceed as in step 4 for the next 3 sensors in the following order:

Right front
Right rear
Left rear
After all sensors have been learned, exit the learn mode by turning "OFF" the ignition.
After the learn mode has been exited, adjust all tire pressures to the recommended psi
After adjusting all tire pressures to the recommended psi, run the tire pressure check in the DIC to see that the correct pressure is listed.