This was the FIRST thing I did when I brought my car home.

Totally easy.

Pushed the outer two buttons until the light begins to flash (to clear the memory of all buttons.)
THEN - while holding your existing remote next to the overhead -
Pressed the desired button on the overhead AND the button of my existing garage remote until the light in the remote flashes and speeds up.

DONE! Worked first time! (Unlike my first marriage).