Hello!
My favourite technique is to use rolled "worms" of blue-tac on the edge of the mask for a feathered colour edge. And you can adjust the diameter of the "worm" - the wider the worm, the wider the colour transition. This can be combined with paper masks cut from copied and scaled instructions. It's just that such paper masks only work for surfaces like wing top or flat fuselage sides. On surfaces like curved fuselage top or wing root I lay out my "worms" looking for specific points like inspection hatches, panel lines and such, and then I fill the surface to be masked with triangles cut from a masking tape - and this doesn't have to be the finest tape as it rarely even touches the model, it's suspended by the "worms".
Then, when spraying, try to stay perpendicular to the model surface, and not spray under the "worms".
And let's not overdo the fethered edge. On a 1:1 Mi-24 Hind in Polish service in the nineties the transtion between colours was 1-2 inches wide. That's not even a milimeter in 1:72, so on a model the transition should be almost invisible. Of course we don't want a "wall" as is sometimes left when you put a thick layer of paint next to the masking tape edge. In the "worms" method there's no such edge, so the colour transitions are always nice.
Thanks for reading and good luck with your painting!
Paweł