1) Show up, tell the nice Red Cross people who you are, and you get a "What you should know before donating blood" pamphlet to read. This will tell you about the (very few) risks involved, as well as all the various reasons why they might not want your blood (HIV, visits to certain countries where HIV is endemic, danger of various diseases communicated through blood, and so on).
2) Next you get to fill out a simple one-page form. This asks for basic identification information (name, address, SSN) and yes-or-no answers to a lot of questions that tell them whether you are a risky donor.
3) Then a nurse reviews the form with you. He or she will also take your temperature, pulse, and blood pressure. They'll stick you in either the finger or the earlobe (your choice) to draw just enough blood to run a hematocrit test. If you pass everything, you get left alone with a piece of paper and two barcodes. One of these codes translates to "use my blood" while the other is "don't use my blood", so if you're a risky donor and for whatever reason you want to donate anyhow you don't put anyone else at risk. Please take these seriously. Stick the correct barcode on your form, the other in the extra box, and away you go.
4) Now you lie down and squeeze a rubberized grip a few times to make the veins stand out. The nurse will clean your arm with betadine, and then inflate a blood pressure cuff (to make the veins stand out more) and insert the needle. Yes, this hurts. No, it doesn't hurt much. It's not as bad as a bee sting or a bad paper cut. You can take it.
5) Then you just sit there and chat for a bit while they take off a pint of blood. No big deal.
6) When the little baggie is full, the nurse will take out the needle and apply a bandage to the site.
7) Now you get your rewards: a little sticker that says "I donated blood today" (wear it proudly!), juice and donuts or cookies or other goodies. Take advantage of them, as you've earned them.
PS - You only have to donate 62 pints to catch me at the moment! Please do!