Press "deactivate account" and then follow the steps to confirm your decision.
Select a reason for deactivating your account and select "Deactivate".
Enter your password in order to verify your action. and click "Deactivate Account" once again.
That's all there is to it! If you change your mind and decide you'd like to be on Facebook again, simply log in to any Facebook app or through the browser and your profile will be just as you left it.