(i would) schedule an appointment in person or through an email. i suggest you schedule it in person, then bring the packet w/ all the info the prof will need (addresses, your resume/cv, why you are applying and why their rec letter would be beneficial to your app. some writers want to know specifically what you want to speak about). some people ask questions to get familiar with the nature of your application and your motives, etc, but if the person really knows you i think there will be fewer questions.
just make sure you think about the obvious questions you'll get beforehand and be clear and concise when describing what you are applying for. also be VERY SPECIFIC with the wording of the request. "would you feel comfortable writing me a supportive and strong recommendation letter," for example.