Close the phone before proceeding.

Using the flat end of the spudger, insert it under the plastic at the top left of the phone.

Gently, although a small amount of force is required, use a twisting motion to pull the plastic casing away from the body of the phone.

Continue to move around the phone counter clockwise until the casing is separated from the body.

It is not unusual for it to pop off before making it back to the starting point.