Letters of commendation are a simple and respectable way to acknowledge someone's good work and to make sure that his superiors know that he is excellent at his job. Although they are quite formal and official, letters of commendation do not need to be intimidating or difficult to write. Since you are honouring someone whom you know deserves it, simply tell the recipient what the person has done to merit recognition. Flowery language and a high word-count are not necessary -- just be honest and specific, and you will have satisfied all the demands of etiquette.
Collect the information you will need. This includes the name and mailing address of the honoree's boss, the honoree's formal job title, details of the actions you want to commend and details of the good these actions have done.
Begin your letter by stating that you are writing a letter of commendation. For example, write "I wanted to write to you commending your business manager, Mr. Horace."
Explain what he has done to deserve your commendation. For example, "Mr. Horace is consistently ahead of schedule, accessible to his staff and prepared to answer our questions. He walks into the office in the morning with a smile on his face, and he sets the tone for us all to have an excellent day."
Conclude by saying why the honoree's actions are important. For example, "Mr. Horace's pleasant demeanour makes us comfortable bringing problems to his attention, and his time management skills ensure that any issues are dealt with efficiently. Our team runs smoothly because of his excellent leadership." Sign with your formal signature.
Send your letter at a time when the recipient will not be too busy or distracted to process it, but whether you are recognising a specific action or event, do not wait more than a few days after the event to acknowledge it.
Be brief, sincere and to the point, and include only positive information. Hand-write your letter on formal stationery to show that you have given it personal attention and care.