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A letter with sloppy grammar and punctuation reflects badly on the candidate as well as on you. Depending on the type of job or course of study to which the candidate has applied, a poorly proofread letter could backfire.  Your word-processing application probably has spelling and grammar checking functions, but you shouldn't rely solely on these. Print a draft of your letter and read over it carefully. Going through it backwards can help mistakes come to light that you might've missed simply reading the letter. You also can read your letter out loud. This can help you find grammatical errors, as well as awkward phrasing or sentence structures that are confusing or difficult to read. If you find yourself stumbling over something as you read aloud, consider revising it so it's easier to read. Your letter should close with a formal closing line such as "Sincerely," followed by space for your signature. You then should type your full name as you plan to sign it. You also may want to include contact information below your typed name, but this isn't necessary.  After you've proofread your letter and are certain it's error-free, print a copy on your letterhead or quality paper stock. Sign your letter using blue or black ink. Make a copy of the signed letter for your own records, as well as another for the candidate's personal records. If you're writing the letter in your official capacity as a supervisor of the candidate, you may also need a copy for the business's records. Find out from the candidate how the letter is supposed to be sent to the recipient, and follow those instructions. If you're sending the letter using email, you may want to send an additional paper copy through the mail as well.  Particularly if you're writing an academic reference letter, or a reference letter for a professional license, there may be additional requirements for the submission of the letter. In some cases you can simply give the letter to the candidate and they will pass it on to the recipient, but in others you must send it directly to the recipient and it cannot pass through the candidate's hands. Regardless, it typically looks better if you send your letter to the recipient directly, rather than through the candidate. This way the recipient knows the candidate has not altered the contents of the letter. Pay attention to the due date. You don't want your letter to arrive late or be disqualified. This could reflect badly on the candidate or cause them not to get the position for which they've applied.
Proofread your letter carefully. Sign your letter. Submit your letter.