Just finished with my visa extension.
Was straight forward for me, I went upstairs and they gave me a card with a queue number (two cards, with the same numbers).
I waiting for about 30-45 minutes and then they start to call out the numbers. They take your passport and one card and then I waited for another 30 minutes, they then changed us to another room and returned with the passports and started calling out the numbers again.
Very straight forward process.
Do make sure you have proof of your appointment by email, as they are checking and may not let you enter without it.