Simply replace the unfilled areas such as "" and "[date]" with your information.
Dear [Hiring Manager],
Thank you so much for your offer of a [position title] position within [Company Name]. I am very flattered to have been given that opportunity. Unfortunately, I will not be able to accept your offer at this time. This position does not fit with my career goals, nor the path I have chosen to attain them, at this time.
Again, I am very grateful for the opportunity presented to me and I am sorry that it will not work out. Perhaps my future career path will lead me back to [Company Name]. You have my best wishes that you may find an excellent candidate for your position.