when I go they give you a form to fill in (health questionnaire) then they take you individually to check over the answers you have given and they prick your finger for a blood sample to check your iron levels.
You then get settled on the bed and then the needle is put into your arm to collect the blood. It is collected in a pouch (I cant actually see it filling due to where it is situated out of my line of sight). It takes 10-15 minutes probably and then after they remove the needle you stay lying down for 5-10 minutes before you get moved to the seating area for your drink and biscuit. You then sit there as long as you need until you leave.
I had a worse bruise on my finger from the finger prick test than I did from the needle in my arm the last time I gave blood! I then went to Asda and drove home shortly afterwards (I gave blood in a donating bus in the local Asda carpark!)
Make sure you have eaten and had a drink and dont do anything strenuous afterwards.