Get the CENTERLINE both sides first .
Maybe blocking & chocking - tapeing / weighting fuselage to a level surface .
Use a offcut or a THING the right height , adjacent , and scribe / scrible along ea. side .
Often pays to mark WING with Fuse. posn. Thus you get LE , TE , & High Point . And you can cross referance .
Back to the fuse , get the LE, TE, & High Point marked . Then away you go , particularly if youve got a curve .
So cutting a cardboard template to fit the wing , at the posn. the fuse. fits . Seperate Fr. & R.
Then if your really worried , Cut another ful length , drawn from those two , and a few dimensions .
Or taped together in situ if youve got 3 or 4 hands .
Thus the templates adjacent the fuselage prescribe the corect cut out .
The more curvacious the fuselage , the better to put pre determined height marks from parallel things ( sticks , squares )and suchlike.
Obviously with the LE - TE & High Point ( Spar ? ) positioned accurately , the rest / curve , will fill in easier .