I bought an Haynes manual for the Focus a couple of years ago, and it has proved very helpful. Pictures say more than thousand...etc.
However I will try to explain in short (from after removing door panel):
-Take loose the interior door handle (2 screws, one under a plastic lid, feel behind, and snap it loose forward, do not detach wire)
-Use a sharp knife to cut through the black glue to remove weather cover upper left part. (careful not to damage the cover, or touch the glue too much)
-Slacken (not remove) 2 Torx screws on the inside of the Exterior door handle/lock (you need an extended torx screwdriver and a headlamp)
-Take your time and slide off the exterior lock cover, and exterior handle (forward movement of handle), and remove the two seals.
-Disconnect the locker cable from the locker unit (connector on the lower part of unit, press clip and ease off)
-Remove 3 larger torx screws from the door rear edge, and maneuver out the door lock unit. (you dont need to detach any handles or wires)
-Put it on a bench under a good lamp, and put your bottle of WD40 to good use. Check operation of the switch (positioned close to the main connector) with the tip of a thin steelwire, before and after cleaning/lubrication.
-All of the sticky grease shall be washed away with WD40.
Good luck !