I like Boss's idea of hiring a pro to do it if you really want it to look skookum. On the other hand, I'm no artist but can paiint a pretty clean, fine line:
1. Good brush. Buy a decent quality artist's brush. No, don't spend $50 (heck, hire the pro for that $$). But don't buy the cheapest one. Buy 2-3 of the medium cost ones and when you practice, decide which is easier to use. Sometimes the fine ones are too fine and you get sloppy trying to make a thicker line.
2, Roll the tip: the technique to making a nice line with a brush is to dip in the paint, and roll the fibers of the brush when you remove the excess paint on the rim of the jar. Alternatively, put a puddle of paint on a nice flat piece of paper (freezer paper, paper side out works great). Dip into the puddle, and then remove excess paint by rolling the brush fibers elsewhere on the paper- remove excess and make a nice fine point at the same time.
3. Brace your hand. When you're making the line, it's critical to set up your work so you can brace your hand so just your fingers are moving the brush. This is absolutely essential. Practice a little and I think that with patience you'll find very satisfactory results.
I worked in a custom sign shop in my youth ("artistic shop assistant", $2.80/hr - I'm OLD!)). I had NO art background, but the commercial graphic artist taught me everything I needed to know in about 10 minutes. Decent brush, roll the tip, brace your hand.
Be close to your work when you're doing it, but assess the quality from normal viewing distances.