I have done this for Birmingham Woman's hospital.
They were incredibly helpful, they sent me out lots of information beforehand to help me make up my mind whether I wanted to do it. They would send me out the pre-sterilised bottles to use, they also sent me a freezer thermometer and a chart to record the freeer temperature twice a day. I know some hospitals have drivers that will come and collect the milk from you, mine didn't but if I phoned them before I left the house they would stay in their office until I phoned then when I pulled up outside the hospital, this meant I didn't need to pay for parking or lug my LO out of the car!
I had to have some blood tests done and complete a medical questionnaire before I could start. I know my hospital ask you to commit to a minimum donation (30 x 100ml bottles) due to the cost of processing each donor. 1 100ml bottle can feed 5 premature babies for 1 day so you can make a big difference!
You can keep donatin until your LO is 6 months because that's when your milk changes, unfortunately I had to stop earlier as I was started on medication for post natal depression.
Any questions, I'd be happy to help xx