It's very easy - turn on the laptop and eject the drive tray and then with the drive tray still ejected turn off the laptop and place it upside down with the screen closed. Underneath the laptop just remove this screw:
Then pull out the drive and at the end in the center will be a piece of metal (sticking out) attached by two small screws. Unscrew this and attach this to the replacement drive and screw it back in. Be sure it put in on the right way round. Push the new drive in, and re-attach the screw.
Then pull out the drive and at the end in the center will be a piece of metal (sticking out) attached by two small screws. Unscrew this and attach this to the replacement drive and screw it back in. Be sure it put in on the right way round. Push the new drive in, and re-attach the screw.