According to the advisors at UT-Austin, the most effective letters come from professors who know you from outside the classroom. This means you need to find ways to interact with professors in other contexts, such as at internships, honor societies, conferences, Student Life clubs, etc., where they can learn more about you as more than just a student in the classroom.
Don't know a professor from outside the classroom? Then choose a professor who knows you well. Most letters are filled with generic praise and don't add unique information to your application. Letters of recommendation are used to show why you should be chosen over other applicants, and the more information the professor has about you, the more effective their letter will be. These letters are more likely to get noticed and have a positive impact on your application.
Most professors have dozens of students - what makes you stand out? Earning a good grade is just the beginning - make it a point to interact with your professors in positive ways. Ask and answer questions in class, email them questions or insights, visit them during office hours. Even in online classes, your goal is for your professors to know who you are.
Choose a professor in whose class you earned a high grade, and who teaches in your major or in a closely related field. For example, if you're an engineering major, an engineering professor should be your first choice, but a math or physics professor would also be a good choice. Remember that a professor may decline to write a letter if you did not earn a high enough grade in their course -- and every professor sets that threshold for themselves.
Choose a professor from whom you've taken a class fairly recently. If it's been more than a year or two since a professor has seen your work, that professor might not be the best one to ask.
Make a formal request by email or in person asking if the professor is willing to write a letter on your behalf.
If you're emailing your request, start with an informative subject line such as "Request for letter of recommendation." After very brief introductory remarks, include the following information in your request:
Your full name and ACC student ID
The semester and course(s) you took with the professor and the grades you earned, or in what other way you know each other
Information about what you're applying for. The more information you provide, the more easily the professor can tailor the letter toward a specific audience.
Any required application forms, with the relevant portions already filled in.
If the form asks whether you waive your right to view the recommendation letter, it's recommended that you do. Letters for students who do not waive their rights to view them are taken less seriously because the letter-writer is less likely to be candid.
The professor may require you to waive your right to view the letter.
You will also be required to fill out a FERPA waiver. Federal law prevents professors from discussing students' academic records with other parties, so students must provide permission to the letter preparer that allows their academic record and achievements to be discussed in a letter of recommendation. It saves time if a completed waiver is included with your request for a letter of recommendation.
The deadline by which the recommendation letter must be submitted. Be sure you submit your request at least two to three weeks before the deadline!
How the letter will be submitted.
Note that many professors will not provide a letter of recommendation directly to a student. They will only agree to provide a letter if they can provide it directly to the institution themselves by uploading it to a portal, emailing it, etc.
Provide additional information about academic and professional goals, including why you are applying for the opportunity. The more the professor knows about your interests and goals, the more specific they can be about why you are a good fit for the opportunity.
Be prepared for the professor to ask for additional information, which may include one or more of the following:
personal statement
Statement of Purpose
academic or career resume
application essay
application cover letter
Here's a Letter of Recommendation Information Sheet that Amazon Web Services uses to guide students who need to request letters of recommendation. While you may not want to give this form to a professor, you could certainly use it to help organize your thoughts!
ACC's Transfer Services has a short guide to letters of recommendation.
ACC's Transfer Services also provides a guide to letters of recommendation for professors. Only provide this guide to a professor if they informed you that they've never written a letter of recommendation, or they've only written a few. Otherwise you run the risk that you may insult the professor.
Don't be overly informal with your professor. Keep the tone of your request fairly formal. The language should be businesslike, and the email should have a greeting at the beginning and a sign-off at the end. And professors are not "bruh", "dude", "fam", etc. Here's a handy guide to professional email etiquette.
Never assume that a professor will be willing to write a letter. Always ask first, even if that professor has written a letter for you in the past.
Don't be offended if a professor denies your request. You may not meet their personal requirements for a letter of recommendation, they may feel they don't know you well enough to write an effective letter, they may feel your skills are better suited to a different opportunity, or their reasons may have nothing to do with you.
Don't harass professors about whether their letter has been sent, unless they've asked you to send them reminders.
Don't forget to send a thank-you note! Remember that the professor took time out of their life to help you, and you may ask them to do it again later. Equally importantly, let the professor know the outcome of your application -- they wouldn't have put in effort on your behalf if they weren't rooting for you to succeed.