How does one approach a professor to volunteer in his/her lab?
Whenever approaching a faculty member relating to research, I recommend that early career students offer them a resume/CV and some concrete example of research interest (a question about a recent paper, how you heard about the lab and why it interests you, demonstrating drive by reaching out to the current lab members, etc). 

In the email, state your discipline, the research requirements, any potential references they may already know (like your advisor). Ask for a meeting with a specific length of time (10-15 min) with a clear objective (to get a tour, hear about the research, get more involved in the lab,etc ), and if you don't hear back within a week -- follow up.