How do I ask a faculty member if I can work with him?
The first thing to do is probably send the academic you're interested in working with an e-mail. In that e-mail, you can say how interested in their work you are, a bit about your background and round off with asking for a meeting to discuss a possible post-doc position?

Hopefully, they'll respond to the e-mail favourably and you'll be on your way. If they don't reply, then perhaps, after a week send a polite reminder or even go chap on their door.

Be bold. The worst they can say is no and that's not that bad anyway, as you'll have the confidence to go on and ask others.