First off, you require 2 keys to do it without a diag tool or your wasting your time.
1. Put 1 key in the ignition and turn the ignition on. (this way the car knows its got a known key in it and is happy)
2. Close all the doors and boot. (The car hates getting a chill and you need to keep it happy)
3. Put your other key in the drivers door and manually lock the car. (Remember the car is happy as it has a known key in the ignition and now you have a second key and manually locked it, it still knows its safe and will accept a new remote signal)
4. Press the button on the remote and you will hear a 'chirp' of acceptance. (This is the car letting you know its delighted to have another remote signal stored)
5. Manually unlock the car. (So it knows not to accept any other signals)
6.Turn off the ignition remove the key, close the door and confirm operation of the remote.
7. Your now as happy as the car 
p.s Your wondering what button to press, theres only 2 and it will chirp when you press the right one.