What is the etiquette for making an appointment to request a letter of recommendation?

I'm currently a college sophomore looking to transfer to a university overseas, and I need a letter of recommendation from my adviser in order to apply.

Usually, in order to get in touch with my adviser, I just sign up for a 20 minute time slot, but that system is usually intended for students to quickly discuss class options.

I feel like signing up for the time slot and asking her for a letter of recommendation in person with no warning is sort of rude, and I've also heard that asking for a letter entirely over email is also rude. So, scheduling an appointment over email seems like the best option, but I'm not sure if I should explicitly reference my transferring and the letter of recommendation, because that seems tantamount to just asking over email. Any suggestions?