If you are trying to fix a chip or a small scratch, read my little post on Langka (it's in this forum somewhere down the page).
With Langka you can be sloppy with touchup paint and still get a nice finish.
If you don't want to use Langka, you'll need a toothpick, or preferably a fine brush from a hobby shop and apply the paint in super thin layers only in the damaged area until the paint fills the crack. If you overfill... you are stuck sanding and buffing and going through hell.
I strongly recommend you to look into getting Langka if you want it to look good.