Dan gives good advice, but Allen told me of similar way, which I think works better than laying the iron on covering about 1/8" from the fuselage, at the wing joint. He told me to use a "Pinked" edge at that juncture. I've been doing that for bout 30 or so years now, and it works very well.
Mask the wing where you want your fillet to end. Use your Favorite method and make your fillet. keep it masked until after your first prime coat. remove he mask and move it 1/8" further out from the first location.
After sanding the first prime coat, paying particular attention to sanding the paint edge down, and scuffing the iron on to the new masking line, apply the next prime, or blocking coat, whichever way you go. Again, take up the masking tape line, and re-apply 1/8" further out onto the wing.
Keep repeating as you apply your fuselage paint coats.
Before the clear coat, do it one more time, after that coat has set enough, remove the tape and set the model aside to cure sufficiently. Use 600 -2000 grit paper and remove the paint line. you can polish the clear coat, and the dull strip, caused by the final sanding, should disappear.
That's what he explained to me, probably in a later stage of his finishing experience than what Dan describes. When I've done all the steps, you rarely can find where the paint ends and iron on begins.