Test the colour first in the door shut on a low verical surface and allow to dry fully, Check the colour match from standing eye level.
This is especially important with metallic touch up paint as the elements can separate over time, this will ensure a better colour match.
Ensure a clean starting area to prevent any contaminants getting into your touch up's.
Remove any excess paint from the brush on the rim of the bottle.
Apply the touch up paint very carefully tring to keep the paint inside the scratch or chip.
Allow to dry fully between light coats (approx 30 minutes depending on conditions). Apply more coats as needed in thin layers until full coverage has been achieved. By applying several thin coats you will build layers that best replicate factory paint.
Deeper chips and scratches that expose a lighter / darker primer (relative to vehicle colour) may require several coats to obtain full coverage.