How do I turn down an offer because I have chosen to go with another one?

Example answer:

Dear [their first name],

I hope you are having a wonderful day.

Thank you very much for giving me the opportunity to work with your organisation in the [job title] capacity. Unfortunately, I am unable to accept the offer at this stage. I wanted to let you know that I have received and accepted an offer from a different organisation. Both opportunities are exceptional and it was a very difficult decision to make.

But I also wanted to say thank you. I am very grateful to have been offered the opportunity to work with you. And I have also thoroughly enjoyed the hiring process so far and getting to know you [include other people from the team if relevant]. It truly does seem like you’ve built an awesome team.

I would love to stay in touch and hope that we cross paths again in the near future.

Thank you again and keep well.

[your name]