I got one today. Scheduled online as well. Before the visit I had to fill out and print out two forms (No. 206 and No. 663), el certificado de supervivencia, and scan my passport. During the visit I had to enter my queue code (which I got online) in the terminal that prints out queue tickets, and then wait my turn to be displayed on the screen in the hall. Then I approached the indicated window, handed all the documents I had, signed whatever I was given and 5 minutes later got my CDI.