[Toughy]

Yes, it's just a cheek swab to determine your HLA typing, however if you are a match it can be a very different story depending on whether it's bone marrow or peripheral blood stem cells (this comes from the FAQ of marrow.org website):

Q: How are bone marrow and peripheral blood stem cell (PBSC) donation different?

A: Donating bone marrow is a surgical procedure done under general or regional anesthesia in a hospital. While a donor receives anesthesia, doctors use needles to withdraw liquid marrow from the back of the pelvic bone.

PBSC donation is a non-surgical procedure done in an outpatient clinic. PBSC donors receive daily injections of a drug called filgrastim for five days, to increase the number of blood-forming cells in the bloodstream. Then, through a process called apheresis, a donor's blood is removed through a needle in one arm and passed through a machine that separates out the blood-forming cells. The remaining blood is returned to the donor through the other arm.