What is the etiquette for making an appointment to request a letter of recommendation?
The reason to want to talk in person is to be able to have the space for discussion and nuance that may be impossible over email.  But you can certainly start the discussion over email.  Why not send an email saying your plans and that you are interested in getting a letter of recommendation, but then close by saying that if the professor is willing, you think it might be a good idea to meet to discuss?