Basic of a Resignation Letter. There's no need to sugarcoat or get creative in the beginning; just state the position you're resigning from and the effective date. While you probably shared with your boss your reasons for leaving, you don't need to describe them here__keeping it simple is perfectly fine.
Prepare for It. Your best option in this scenario is to accept it with dignity and make sure you're ready to leave the office for good when you turn in your resignation letter, whether that means gathering up your personal items or making sure you've collected the names and contact information for the teammates and clients you'd like to stay in touch with.