they take your cells in one of two ways
1) operation where they anaesthatize you and harvest the bome marrow from your pelvic region (no cutting involved, but can be very painful afterwards) like any operation, this carries risk
2) they give you a weeks worth of cell growth factor injections which makes the bone marrow stem cells proliferate and spill out into your blood. You then have them spun off of a machine, a bit like giving bloog over a couple of days (or maybe 1 day if they get enough)
with this method there is also a risk because no-body knows the log term effects of the gowth factors