First, make sure your MacBook is laid on a soft cloth preferably on the floor or a carpet to prevent it from sliding or falling.
Open the MacBook top cover and be careful not to scratch the screen. Now reach in the top corners of the MacBook screen and pull the hooks away from the corner one at a time.
You might want to use a piece of stiff cardboard / credit card instead of your fingernails. Just be careful not to scratch your MacBook or break your MacBook plastic case. The case should slide off easily.
Repeat the same for the bottom cover of the MacBook. Remember to remove the hooks one at the time.