I'd say it depends on the complexity of the camouflage.
For those which consist "only" of more or less uniform areas of different colours - such as those schemes used by the RAF - it's usually sufficient to either gently trace the outlines of the various areas with a sharp pencil, or to use masking film cut to shape with the appropriate contours. This, of course, also depends on whether the camo scheme features sharp or faded edges between the various colours - with masking film being the better choice for sharp edges.
However, I personally wouldn't worry too much about precisely hitting those specific outlines which are usually illustrated in the instructions. Considering the truly huge numbers in which many WWII aircraft were built and the equally large number of personnel which I assume will have worked in the paint shops alone over the years, I guess it's fairly reasonable to assume there was a considerable margin for deviation here - even when there were official diagrams of some kind or other which specified how the various colours needed to be arranged. In the absence of preserved examples of such diagrams, however, I would think trying to copy down camouflage outlines from prototype photos of the time - a fair number of which will of course lack the quality of modern day photographs - is very much bound to entail the same kind of margins.
However, let's now take a look at a WWII Luftwaffe plane, such as this Heinkel He 219 night fighter: Click. This kind of scheme can basically be done only freehanded. That's one of those examples for hideously complicated schemes I was referring to, by the way!
As a general rule, which I think you'll probably know already - in most cases the lightest colour needs to be applied first and the darker ones go on top.