Key fobs not only lock and unlock your doors, many of them also turn your alarm system on and off. You'll need two simple pieces in order to program your fob: all the fobs you want to program and the manufacturer's programming instructions. Programming the remote keys to your car can be done in less than 10 minutes and does not require any real knowledge of a car's system in order to complete.

Programming

Start by obtaining your key fob whether you have bought a new/used one or you need to reprogram your old one. When you program a fob into the car, any information on the old one will be lost and allow for a clean reset of the fob into your programming system. This is good for people who cannot afford a new OEM (Original Equipment from Manufacturer) and need a used key fob. You will also need to obtain your manufacturer's programming mode setup instructions. You can look up all a list of the major manufacturers online, obtain the information from your manufacturer's dealership or visit an auto parts store, which also often carry the information for free. With these instructions you can now return to your car and begin. Programming the fob will always go in the same order: 1) Enter programming mode, 2) hold down buttons on the fob to recognize the fob and program it, and 3) exit the programming mode. This will always be the succession order. Now keep in mind that every manufacturer is different and will have a different setup for the programming although those three processes will all be in the same order. The first process, entering programming mode, will consist of inserting the key into the ignition and an order of instructions. Upon successful completion of the instructions, a signal that will let you know you entered programming mode, such as the doors locking and unlocking or the horn honking. This is now where you enter the second phase---programming---which will ask you to press a specific combination of buttons on the fob. In programming mode, you can program up to four fobs to the car within a 10-minute time limit. Use the instructions to press the buttons, usually for about 10 seconds. You will receive another confirmation in the form of the doors locking or another horn honk. Now that all of your fobs have been programmed to the car, the last step is to exit programming mode. Exiting the program mode is usually the simplest phase. This will consist of something simple such as removal of your car key from the ignition or starting the car. Once you've finished this step, you can test the remote to see if your fob is now programmed.

Source:

Program Your Remote: List of Manufacturers and Makes