Sent two decline e-mails after I accepted an offer from another. One program accepted me, another reached out to touch base.
Kept the e-mails short and professional. Thanked them for the offer, stated how it was a difficult decision, mentioned that I wish to work together in the future. The program that accepted me wrote that they were disappointed but understanding. Had me fill out a survey on why I chose another program (not their business tbh) and how they could improve. The other program thanked me for my response and wished me well.