If the laboratory has a deadline, then about 15 minutes before the end of the laboratory you should announce that those who have not completed should obtain the extension from you. Wander round the room and ask them to raise their hands if they need the extension. Some people get the students to queue up for this, but it's surely more efficient and less disruptive if you do the walking!
It might be tempting to simply give the extension to everyone present who hasn't completed. Indeed you should do this if you or the laboratory has had unforseen problems and there is no time left to do it properly. However, the usual process of the students asking for the extension is an important part of the psychology used in ARCADE, especially for first year laboratories: the students know they must finish the work in their own time by the start of the next laboratory.