There really is no easy way to line a fretless board. Any paint/pen/laquer you could think of will rub straight off. The only way i to do it properly with quarter lines or full lines with wood veneer/maple or other hardwood. You could paint them on and then epoxy the board which is an art in itself. The online fret calculators can tell you where they need to go, Stewmac have one on their webite. Whatever you do, DO NOT try and laquer a fretless board with rattle cans. It doesn't work.