My experience:
I would approach an instructor about writing me a rec. They would tell me what they need from me, usually background info like my resume, who to address the rec to (I usually ask for just a general rec addressed to no specific institution), and ask to have the rec sent to my school's pre-dental advising committee (for inclusion into the composite LOR package).
Seems to have worked for me...
You can go the extra mile and ask the instructor for tailored letters addressed to the specific schools you are applying to too. Depends on your situation I guess.
Good luck!