I know this is a bit late mate but in case its needed again. Theory is really good but in practise:
I usually preheat to 150c weld with hydrogen controlled electrode flux 71 or 81. Dual shield or
7016 -18 sticks. Dont exceed 250C interpass, so pien with needle gun and it should be cool enough to go again. When welded let cool. That easy. The preheat is so it doesnt cool too fast and trap diffused hydrogen, helps dry atmospheric condensation (you will see it get wet when you heat it) and thick sections steal heat from weld zones. You can cover it with a blanket if you need if you live somewhere very cold or post heat but otherwise you dont really need to. Ends of welds and restarts should be grinded a bit neatly and peined just cause thats a nice pro finish.
Hope it helps mate