Such letters are customarily written when declining an offer from a prospective employer, as it is courteous to turn down a plant trip or job offer graciously.
- Write the person who wrote you.
- Thank her/him for the offer.
- Briefly state that you are declining and why. Remember not to get too personal.
Never copy examples, inserting details. If companies receive identical letters they may question who is doing the writing and your sincerity.