How to create a soft edge with spray paint?

Since the airbrush already leaves a fuzzy edge, if you can freehand the camo, that automatically gives a soft edge.  If the patterns are very standard, and you feel you cannot do the patterns freehand, there are two methods.  One is to hold patterns against model and trace edge with soft pencil or marker close to the color you will be spraying.  Fill in over those lines, then fill in the solid area inside the edge lines.  A second method, which takes more care, but protects the areas where the existing color remains, is to tape down patterns that mask those areas.  Then, when airbrushing, do not take the paint right up to the line.  You must be careful with this method because if you get to close to pattern edge it will make a hard line in that area.  But the advantage of this second method is that area to remain in existing color is not at risk to accidental painting.

EDIT: as here Testors tutorial no 5 & no 6

Then spray away from the edge. better described as above links.

But this technique is difficult to do with rattle cans? 

make several photocopies of the camo layout page to the same size of the plane,

the newer Tamiya plane layout page is already the same size.

Decide which is your base colour, & spray the plane.

Cut out the curves so that you have masks, several smaller parts taped together will go round the fuselage better.

What you need to do is create a 'stand-off' of the mask. you can either roll thin 'sausages' of poster/white/blueTac & inset several mm to the edge. but it's not easy to get a consistent  hight of the mask to the surface. as here

Another option would be to use thin cardboard or several thickness's of  tamiya tape to create the stand off