I have been called on to be a Stem/Bone marrow donor, what happens next?

The center you have registered at will guide you through and arrange everything.
When you are matched to a patient you will be contacted by their medical team and they will guide you through each step. You will be requested to have a blood test at your local GP or hospital.
You will have a further medical assessment and consultation at a specialist collection center (where you will later donate your blood stem cells).
If you are healthy and fit and happy to donate your stem cells/bone marrow then this will happen one of the two ways:
Peripheral Blood Stem Cell (PBSC) Collections: Happens 90% of the time.
Bone Marrow Donation: Happens 10% of the time.
Peripheral Blood Stem Cell (PBSC) Collection
This method is used 90% of the time.
You will receive injections of a stimulating factor called granulocyte-colony stimulating factor (G-CSF) for four days prior to your donation. This is simply to stimulate your blood stem cells in the bone marrow into the blood stream ready for collection. These injections are administered by a nurse at either your home or work.
For the actual donation, a needle is placed into one arm and your blood is circulated through an apheresis machine, which acts as a filter to remove the blood stem cells. A second needle then returns the blood through your other arm.
The whole process takes about approximately 4-6 hours and you can return to work within one or two days.
Bone Marrow Donation
Bone marrow donation is a surgical procedure that takes place in a hospital operating room. You will be given anesthesia and feel no pain during the donation.
Doctors use needles to withdraw liquid marrow (where the body's blood-forming cells are made) from both sides of the back.
The side effects of this (if any are felt) are minimal: you may feel tired for a few days to a week: some compared it to the feeling after a grueling rugby match or the feeling before the onset of flu.
Please note that your stem cells replenish themselves very easily. Therefore donating your stem cells does not put you in any risk.
Here is a short video from Anthony Nolan to explain all of the above: