Open the armrest. Remove it from the car by taking out the screws visible in the picture below. Separate top and bottom halves of armrest (screws on the underside of the armrest.) Install new leather OVER the factory one - test-fit it carefully first, then glue it using Automotive GOOP, 3M Super 77 or any other upholstery glue, making sure you follow the instructions on the glue container and making sure the glue you pick is not an instant glue, so that if needed you can adjust the fitment before the glue has cured. Once you have it installed nice and straight, put the two halves of armrest back together and install back in the vehicle.
