Introduction/Opening: Begin your letter with the bad news. The indirect approach includes a buffer, however, the direct approach does not. Be sure to remain professional and concise in this section to not sound rude or ungrateful for the opportunity.
Body/Discussion: In the first paragraph of the body include your reasons for the decision, without too much detail about the other position. Having the right balance of ambiguity is respectful to the employer you are declining. Use creativity when developing reasons for taking the other position over theirs. Maybe the schedule was more flexible or the other position better fir your skill set. These are just a few examples to consider when writing this section.
In the second paragraph you can add a positive spin to the situation if you desire. This could be as simple as saying you’ll keep this employer in mind for future endeavors or that you will recommend a qualified friend who should apply for the position.
Conclusion/Closing: Begin your closing paragraph with a courteous transition and thank the employer for the opportunity. Conclude your letter by indicating how the reader may contact you if he has any questions.