There's a great article in the Osprey Modelling book, Modelling the Panzer IV in 1/72 Scale, by Alex Clark. He basically rolls the milliput flat between wax paper, carefully peels it off the paper and applies it to the sides. Then he trims it and uses water to smooth it over. He then uses a stamp with the proper pattern.
I can't imagine trying to do zimmerit with Squadron green or white putty. Maybe a small corner where two sheets of Cavalier zimmerit come together, but that's about it.