I love that soft edge in the 2nd photo! Was that done with putty or just freehand?
I’ve used some kinds of adhesive similar to Blu-Tac and it left an oily residue on the surface 
Thin out your paint at least 1:1, or even greater (practice), lower the air pressure to 10 psi to start with, and hold the airbrush nozzle almost touching the surface (again, practice on scrap), and get a line as narrow as you can - 2 - 3 mm’s wide, and paint the outline of your pattern. You can make the line with a very fine soft edge, then fill in. But…practice - practice - practice!
Freehand, using the technique @Biggles50 described. Had the airbrush tip removed and pressure around 8psi. Took forever! 
Tape some Index Cards, Cut Out the Pattern
Been tryin for three years and still cannot pull off hard or soft edge. grumbles
What type of airbrush do you use ? When I remove the tip from my Badger 150 it doesn’t spray at all.
The Badger can be “tuned” (abominated) to do this.
Part 50-0371 /..72 /..73 is the tip in question BUT it also provides the small opening around
the nozzle, part 50-0391/..92/..93, which creates the required airflow around the nozzle to
make it spray. If the the tip is left off the airbrush will not spray.
The tip can be converted by filing down the front to remove the raised circle.
This will however expose the tip of the needle and the nozzle to damage by being rammed
against hard objects. This modification can not be undone so it is advisable to buy a second
tip and only use the modified tip for special jobs.
The part number triplets are because these parts are different for different needle sizes.
Other airbrushes have a different design for the tip.
The front end of the Badger Crescendo looks like this:
and allows getting closer to the surface to be painted.
Check parts diagrams for other airbrushes, or get a spare tip for your 150 …
Many thanks for the thorough answer, have you done this yourself ?
No. Not yet.
Might happen but maybe one of my other
airbrushes have another design so that I don’t need to.
Need to rebuild my compressor first …
The AK putty is great, however it’s expensive. Theraputty used by occupational and physical therapists is the same thing and a whole lot cheaper - even more so if you know a PT or OT, if not you can get it on Amazon.
I’ve had better experience with patafix than with blu tack. Blu tack is more like chewed bubble gum (stuck to the bottom of your shoe) than any substance I’ve encountered. Patafix after it gets old will start to behave similarly but I get a lot of use out of it before it gets there. And it holds better when it’s new.
YMMV
I use a similar one from Ammo, in any case these are tested products specifically intended for modelling use. Other products for other uses may, or may not, work -note that there are dozens of brands for silly putty, theraputty, blue tack alternatives, and they may behave differently. What worked fine for others may not work the same for you.
AK or Ammo can be more expensive, but considering your model will be safe, for me that alone is worth. And being reusable, it lasts for years so personally I do not care expending a bit more.
Andy HHQ has a similar product.
He had the other two products (currently out of stock) and this one was cheaper.
I wasn’t clear. Just removed the guard on the end, exposing the needle.
Never tried @Uncle-Heavy idea.
It depends on how the airbrush is constructed.
On some the guard is only a guard,
on the Badger 150 it is a guard AND a necessary
part of the spray function