Make a small dot in the middle, near the top of the muzzle (see photo). Use black gel (paste) colour dissolved in a very small amount of water and a very fine paint brush. 

Don’t apply too much paint on the brush (take the excess off), otherwise it will be impossible to paint thin precise lines. If the paint is too thin it means you added too much water. If the paint is too thick (you are getting broken lines), that means that you need to add a little more water). 

In order to steady your hand place the palm of your hand on the board/table. Don’t touch the head otherwise you will indent it. If you are unable to hold the hand steady without touching the head, leave the head to dry completely before painting the nose and mouth.