Crunchymum - you go in, either with an appointment (best to do this as otherwise you may wait for ages or not even get seen at all if they're busy) or just walk in. They'll give you some forms to complete, asking you things about your medical/social/travel etc history. You may also get given a big glass of water to drink while you wait.
Then you'll go and talk through the form with a nurse to make sure there's no reason you can't donate (risks of transmissible diseases, any medication that might not be suitable etc). They'll do a fingerprick and take a drop of your blood and put it in a coloured solution to check your iron levels are high enough to donate.
You'll then wait again and be called for your donation. You are kind of semi-reclined either in a special chair or on a trolley. They put a blood pressure cuff around your arm and partially inflate it, feel for a vein, clean the skin and then put the needle in. It can be a bit sore but once it's in it's ok. It's connected up to a very clever system of tubes and bags, so they can take a couple of vials (3/4 I think) to test for blood group and various diseases etc, and then the rest goes into the main donation bag. They take just under 500ml in total, it usually takes 5-10 minutes, and I think they have a cutoff of 20 mins.
Once it's done, they'll take the needle out and put a dressing on, you'll usually sit there pressing on the site for a few minutes until it's stopped bleeding. If it's your first time they'll usually get you to stay lying down for a few more minutes before getting up to reduce the chance of fainting. Then you go and have a cup of tea and a biscuit, and get a sticker!
I think I've given about 18 times, keep having to take periods off due to travel/needlestick injury, and currently because I gave birth less than 6 months ago, but I'll get back to it once I can!