I've done a lot of paint touch-up and can offer a few tips.
1. Make sure the area is clean. Paint mixed with dirt -> not good.
2. Magnify!!! Find a powerful magnifying glass/lens. I use one with an integrated light to eliminate shadows.
3. Dab, don't brush. If you think a brush stroke is needed then you applied too much paint...
4. Use as little paint as possible! Three or four thin coats are much better than one thick coat.
5. If the scratch is very narrow, purchase a micro-bristle brush at hobby or paint stores. They're not cheap but worth every cent.
6. Breath. Don't hold your breath while painting. It can cause your hand to shake and reduce eye/hand coordination.
7. Paint a small portion of the scratch, step back to relax, then come back as needed to continue. Baby steps.
After the paint dries for several days, come back with some very(!) light-duty rubbing compound to knock down roughness. High grit sandpaper (4000) also works but I don't suggest this unless you have plenty of experience. If using sandpaper, mask the area first to prevent further issues.
Finish with a hard, high quality, high shine polish or wax.