Good links there. A 1/72 Spitfires camouflage would appear hard edged as there was only very slight (and regulated) feathering on the full size machine. If you don't fancy trying to cut paper masks for the rather complex curves of British camouflage I'd try using good old Blu-Tac. Here's what I do,main colours,not wheel wells,canopies etc.
1 Spray undersides (for you that will be Sky or Medium Sea Grey, unless you are doing a very early one)
2 Mask undersides
3 Spray upper surface in lighter of two colours ( for you that will be Dark Earth or Ocean Grey)
4 The fun bit! roll out some skinny sausages of Blu-Tac and lay it on the wings and fuselage following the demarcation lines of the camouflage scheme. You can move them and fiddle as much as you like until you reckon you've got it about right. You can press them down on the surface, as for a hard edge you are not looking to "space" your masks from the surface. Do remember that the area under the Blu-Tac will become part of the DE or OG area.
5 Mask inbetween the sausages,with bits of masking tape, the area that you want to keep DE or OG.
6 Spray your second colour ( this will be Dark Green in both cases). Don't worry about keeping at 90 degrees to the masks as you want a hard edge. Don't spray to much into the edge of the mask or the paint will build up and give a ridge which will be a b##ger to get rid of.
6 The denouement. After a suitable wait (depending what sort of paint you use) remove masking and Blu-Tac to reveal perfect camouflage.
It really isn't very difficult.
Good Luck.
Steve
Top Tip: Wash your hands before using the Blu-Tac.This will prevent it leaving a slightly darker residue on the demarcation line. Some people intentionally allow this "darkening" as they think they can discern this area on the originals. Me, I am enjoying myself on this fence!