Stand by front wheel looking towards the back of the car, with the mirrors in the normal "out" position.
The bottom of the cover from the outside working your way in, get a flat thin (non bendy) object and work its way into the gap, between the cover and the rest of it. Angle downwards at first until its in the gap then push the handle of that object down (so it points up inside the cover)
Use your hands/thumbs to push the edge of the cover up and in towards the side window in order to create a bigger gap between the cover and the rest of it to work the thin metal object you've got to get in better. You might even be able to get your fingers in around the far edge bit.
Then just force it off using that object / hands pushing, (support the rest of the mirror from underneath at the same time if you have a spare hand.).
Its horrid trying to do it, because your convinced its going to break it off (it didnt for me - but i cannot promise that yours wont!)