Summarize this article:

Let the professor know up front that you are requesting a letter of recommendation. This tells them your email is timely and lets them know what to expect from your email. Your subject line might read, "Request for a letter of recommendation for graduate school." This makes it easy for your professor to read your request. Don't put the request in an attachment, which makes it harder for your professor to open and read it. No matter how well your professor knows you, it's unlikely they'll remember everything about you. Your professor will be able to write a better letter if they have a list of your achievements, work history, and educational background in front of them. Attaching them to your request allows your professor to review them along with your request. You might also attach samples of your work and a draft of your application essay. This will help your professor tailor your letter to your application. Sending in your letter will be easier if your professor knows exactly when the letter is due and where to send it. Provide the address for a written letter or the digital link for an electronic letter.
Use a professional subject line that clearly states your request. Include your written request in the body of the email. Attach a list of accomplishments and your resume or CV. Attach a list of due dates and where to send the letter.