I will be writing about this topic in the context of applying to college, but if you are ever planning to (or thinking of) apply to a summer program that requires a rec letter, definitely read this!
Recommendation letters are a HUGE part of your application, whether you like it or not. A rec letter is one of the only 3rd party perspectives admission officers will have on you, and you want that perspective to be as positive as possible. Admission officers will look to these letters to see how you operate in a classroom setting, whether you demonstrate intellectual curiosity, your involvement in the school community and beyond, and much more. You can read about this in detail at the Common App's website. FYI: rec letters are often referred to as "support" in admission files, like the comments made in my admission file, demonstrating that these letters significantly impact your chances of admission.
So, how do you get a rec letter and whom do you ask? To get a rec letter, simply ask. If you're applying to college in the fall, it's a good idea to give your teachers ~2 months before the deadline to write the letter. You can get a rec letter from three main sources: (1) your college counselor, (2) one of your school teachers, and (3) an instructor or coach from your extracurricular activities. Try to diversify your rec letters. Ask one STEM teacher and ask one humanities teacher. This will demonstrate to colleges that you are a well-rounded candidate and well-versed in all subjects.
Incoming freshmen, sophomores, and juniors, be sure to start (if you haven't already) building a strong mentor-mentee relationship with your teachers. This will make it much easier down the line to ask for a strong rec letter. If you're not sure how to build a strong mentor-mentee relationship, check out the video I posted (linked below) where I provide specific steps you can follow. Building a good relationship with your teachers is a *very* important skill to have, even in your adult life. When you get a job, you'll have a boss and managers to report to. The better your relationship with them is, the easier your job will be.
After you ask, it's always best practice to send your teacher information that will help them write a strong recommendation letter for you. Examples are your resume, drafts of your college essays, or a snippet of your arts supplement or athletic feats. Sometimes, a teacher will even ask you to complete a brief form with questions about you and your interests or invite you to a quick coffee chat (or in today's case, a Zoom chat).
After your teacher has submitted your recommendation letter, be sure to send them a *handwritten* thank you card. This doesn't directly increase your chances of college admission, but building the habit of saying "thank you" will lead you to success and help you maintain a strong relationship with your mentor. Be careful not to send them the thank you note *before* they have submitted the rec letter, or else your note and any gifts you include may seem like bribery (yikes).
I shared more details (including a template email for asking for a rec letter!) in my latest YouTube video. Be sure to check it out!