First of all, what scale is the model?
Anyway, that makes little actual difference insomuch as the technique is pretty much the same, it's just harder on smaller models.
Even though it sounds easy in principle, painting a (straight) line on a model a/c is a long and tedious procedure, and should not be attempted if you are careless or impatient.
Basically, all it entails is masking off a straight line with two pieces of tape, applied to both sides of the fuselage. Space them however far apart you need the line to be. This should be done BEFORE you attach the wings. You need to "eyeball it to make sure that the gaps are of equal "thickness" on both sides of the fuselage, and that the tape is straight and level, with respect to the windows. Otherwise, you'll end up with an old TWA livery type stripe. And I don't think that you want that on a Lufthansa. Then paint the exposed part of the fuselage. Wait at least 24 hours for the paint to completely dry, then SLOWLY peel off the masking tape. If you did it right, you should have a nice straight line. But a word of caution, try and avoid painting the tape as much as you can. Otherwise when you peel it, it will flake to the tape, and when you remove it, you can find yourself with a nice segment of paint missing from where you want it to be.
As for the nose, and the tail sections, that is the toughest part of all. Unless you are good (or have the patience) at cutting curved sections of masking tape (involving taking accurate measurements of the fuselage), you will have to VERY SLOWLY and methodically paint it by hand. I once built a Reno Air 1:200 MD-87, and I could not get the decals to properly cover the nose, so I mixed paint until I had the right shades, and painted the nose stripes on.
Those two little stripes, each only about 3/4 of an inch long, took me nearly an hour each to do.