Keith, the head of the program, based in Minnesota, Dr. Dennis Confer, is a good buddy of mine. Basically, they drill a hole into your hip bone and suck off a unit of marrow (about the size of a unit of blood ~ 500 cc), treat it, and freeze it in liquid nitrogen. You're anesthetised, (usually concious sedation, kind of like for a colonoscopy) and when you wake up, you might have a little bone pain. It might last a day or two, a dull achey pain. It's really not bad, although I'm speaking from the standpoint of the guy who does bone marrow biopsies. They usually do it in the OR to assure sterility. The marrow is treated with DMSO to preserve the cells and then snap frozen at -40 degrees and stored for transport.
My hat is off to you for volunteering to do this, it is very humanitarian. I have a rare blood type, so they really never want mine, just my plasma.