Your acceptance letter should be short -- just a few paragraphs -- and to the point. You want to thank the person who hired you, discuss the job specifics and quickly restate the assets you will be bringing to the position. Your goal is to reinforce to your new employer why he or she chose you. Close the letter by letting your employer know once more how much you appreciate the opportunity.
The tone of the letter should be upbeat; it should convey your excitement about embarking on this new opportunity.
Once you're finished writing the letter, check it over carefully to make sure there are no spelling or grammatical errors. Be especially careful about the spelling of names. You definitely don't want to misspell your new manager's name.
Make a copy of the acceptance letter for your files. Then send the letter via certified mail (or use a tracking service) to make sure that it has been received.