If you still attend the school or institution that the teacher works at, it’s appropriate to ask in person.
However, if you don’t see this instructor on a regular basis or haven’t communicated in a while, email is your best bet.
In the event that you lost this particular teacher’s contact information or it changed because they moved to a different school, you can contact the school they work for to find it or try to see if they have a professional social media page like LinkedIn you could use to find their email or reach out.
Try to avoid contacting them in personal venues such as by telephone, at home or through regular social media — that could come off as pushy or invasive.
If they do agree to write a letter for you, you can share our How to Write a Recommendation Letter guide to make the task even easier!