The procedures vary from manufacturer to manufacturer and even between different models from the same manufacturer. I suggest you call the phone number for the manufacturer of your unit and ask for tech support. You will need to know the model number, so check out the nameplate before you call.

Here is my problem. Craftsman 1/2 hp model 139.53675SRT2, vintage 1998. Wall control works fine, homelink in van works fine (99% of the time), keypad was getting very erratic. Bought new keypad model 139.53876. Following the instructions, I select the new code, press and hold the enter button on the keypad, and press the SRT button on the opener body which is the sequence for the opener to learn a new code. At this point the instructions say that either the opener lights will flash or the door will move. Mine does nothing -- no reaction whatsoever. I proceed to try the same process on the other opener (I have two garage doors, the second does not have an existing remote keypad). Same (non) result-- no reaction from the opener. Comments?

Try pressing and immediately releasing the button on the back of the motor unit and then enter your 4 digit PIN and press enter. If this doesn't work you most likely got a defective keypad. I would take it back and get another. Be sure to take the model number of the opener with you so that they sell you the right keypad.