They will let you thru the turnstile, and then you will go to the left, at a little office just past the restrooms. I believe it's called 'Park relations' or something like that. They will direct you tho, no worries. When I did it last August, it took about 20 minutes, as there were 3 people in front of me. It was so great to get my little photo card back.
I could have come back later to do it, but I was anxious to just get it over with.