You can apply to be on the bone marrow donor list when you give blood, or online at the Anthony Nolan Trust website.
You may never hear from them again, or they may ring you and say they've found a matching recipient. There are a few preliminary checks in which they make sure you are healthy enough and explain how it all works. Even if you are on the list, you can pull out at any time, but bare in mind it costs them money to register people, so don't register without giving it some serious thought. There are two ways in which bone marrow can be donated.
The more common method is to hook you up to a machine that takes blood from one arm and puts it back in the other, but removes the part they need in between - rather like a platelet donation machine does. You are awake and if you've given blood you'll know it's not too scary. This also involves a course of injections beforehand, to make sure you have optimum amount of bone marrow available. A nurse will come to either your home or place of work to administer these, so no need to have time off work in advance.
The less common method is to give you a general anaesthetic and to remove the bone marrow by sticking a needle in your hip. Sounds fairly horrific, but you will be asleep and it's only a tiny dot of a scar and possible soreness.
Both methods have a recovery period of no more than a fortnight, but you will have to stay in hospital overnight if you have the anaesthetic. Which method they use is decided on a clinical basis (I'm not sure how) as clearly the hospital would rather not have to put you under if possible.
You can read loads more on bone marrow donations at the Anthony Nolan Trust website. It doesn't matter if you register via them or via the NHS donor service - the databases are all combined to find the best possible matches for recipients.
Lesson over for today!