The lightest and fastest (other than drawing them on) is using 1/64 and 1/32 chart tape, just glass the plane, sand to a smooth finish, get out the three views, wipe the plane down with windex, and put the lines on the plane where they are on your three views. Cut the tape even with itself, do not overlap. Then just paint right over the tape leaving it on. This saves weight by not having to do any primering, but the panel line is a "raised " line, and it will look great from 3-5 feet. The penalty here is, If you do not seal the line with the paint it may lift it(after many flights). Doc Keith used this on many of his planes,

# 2 way is to put down the lines the same way, primer over them, sand up to the line, then pull up all the tape and then paint the plane, now you get a panel line thats inset into the paint, also looks great but there is a weight penalty. if you are going to do rivets also this is the way to go.