Panel lines, how to make them fresh?

Say you're adding a body panel. Or a door. Or a hatch in, say, a scratchbuilt fastback. How would you scribe the panel lines? I tried freehand but they're all wavy. I can't use a straight edge because the edges aren't straight. How is it done? I've seen piles of well cut panel lines custom made in models all over the place. Especially on slosnipe's immaculate Civic sedan. What's the secret? Templets? French curves? Magic? I'd like to know so I can fill the lines and give it another, better, go.

take some thin plastic sheet, cut it in a straight line or a line to match the new door, tape it down where you need it, and then run a very sharp Xacto along the edge of the plastic, it should allow to have a very straight line,

I have a flat Xacto about 2cm wide..

it might be wavy, but only every 2cm :D

There are scribing templates that military/aircraft modelers use because many of those kits have raised panel lines instead of engraved.

They have templates for curves, 90 degree corners, straight lines and rounds.