An apology letter is usually written in both personal and professional life on occasions where the addressee may have faced inconvenience because of you or a person or entity represented by you. There are a few general tips that should be kept in mind while writing an apology of any nature.
The most important point to be kept in mind is that an apology should be sent as soon as it is possible to do so. The tone of the apology should be considerate and should be very respectful. Choose words carefully and use simple language which shows sincerity while writing these letters. Accept full responsibility of the inconvenience without giving too many excuses however genuine.
Try and keep the letter brief with focus being on the solutions or rectification measures to taken by you with assurances that such occurrences would not repeat in future. Also, if possible mention a time frame where in you could apologize in person. But this needs to be strictly adhered to.
In case, of a professional apology in the event of poor service, defective product, missed deadline, delayed delivery or delayed payment, a timeframe for the solution, the steps taken and the current status of the process should be clearly mentioned.