Well I assume it's like any PhD. Contact a PI by email on a professional level. Say who you are, what you have done, and what you are interested in doing. If the school is nearby, ask if you can come at a mutually convenient time and tour the lab and meet people and get a chance to talk to the PI and see if you have similar goals. At the very least, ask speak to the PI on the phone. Be prepared with a list of questions about the lab and about the work they are doing. Show that you have researched ahead of time and you know papers that the PI has published, etc and why you think you would be a good fit for the lab. Show that you are motivated and proactive. If the PI roots for you, remembers you, and thinks you're worth his time and you meet the other qualifications, usually you are in (at least on the phd side of things....i cant speak for the dvm side of things).