After the 12th rep, you put the dumbbells down and take a short break (say, 30 seconds), have a sip of water, walk around a little, shake out your arms... then you do another set of 12.

I have a question which I am not sure if I should post it as an entry, so I'll try asking here first: Right now, I always do one set of each of my exercises in a row and then another of those 'supersets'. Should I rather do the sets directly after each other? I figured the additional rest in between the sets would be good, but it sounds as though other people do it differently.