The most important aspect when declining a private school’s admission offer is to be respectful and timely. The school may have specific instructions for declining admission, but in general, you want to put it in writing, and an email or formal letter typically suffices.
Timeliness is of the most importance because your decision could open an opportunity for another student to attend the school.
Keep the letter short and concise, and make sure to thank the admissions team for their time and consideration. You don’t need to go into detail about your decision, but if you are comfortable briefly explaining your rationale, most schools appreciate feedback. In fact, they may ask you to complete a follow-up survey for their data tracking and internal analysis.