I did it back in college and tried to do it again back when I lost my job about 2 yrs ago...the second time around it didn't work out too well, I kept being denied because this or that was not in the guidelines (iron or protein levels-I don't remember now) so I gave up.
At the center I went to you first had to schedule a "physical" with their nurse. At that appointment they went over a ton of health questions to be sure you qualified to give.
The normal donations usually took 1 1/2 hours, give or take. First you answer a series of questions (things like have you ever taken a certain drug, have you received ny immunizations since your last visit, are you feeling well today, etc.) then they weigh you, check your temp, pick your finger and check levels of certain things in your blood, look at your arms for bruises. Then, if you are all good you go and actually donate.
Donating is a lot like donating blood, except the red cells are sepaarated from the plasma and they return the red cells to you. They also give you some saline at the end.
I think the pay around here was $20 a visit, but if you went twice in the same week the second visit paid $30 and if you donated twice a week for the whole month you got another bonus (maybe $250 per month). Then there were other programs like get a tetanus booster and donate so many times after it to receive a bonus and things like that.
Our center even had a child care center included for free.
It was not a lot of money and did take some time, but a decent menas to earn a little extra cash. It just didn't work out for me. Between failing my finger picks and them not being able to find my vein in my left arm--it was a PITA.