Dead easy bud.
1. stand behind the mirror glass facing away from you.
2. Hold onto the skull cap with both hands
3. Horizontally pull the cap towards you.
4. If a little persuasion is required you could use the palm of your hand to strike towards you on the edge closest the car.
There are two main clips inside which hold the cap in place and if they have never been removed before you will defiantly need point 4.