Once the surface of the paste is firm, draw a smile with a fine, edible marker. Don’t press the tip too hard to the paste. If the paste is still soft, leave it to dry for longer before making the smile.
If you don't have an edible marker alternatively you can:
- paint the smile with gel (paste) colour dissolved in a tiny amount of water (use a very fine brush) or
- roll a very thin sausage and stick it on (I suggest using flower paste).