Here's my royal icing recipe:
1 1/2 cups (240g) pure icing sugar
1 egg white (from large egg)
1/2 teaspoon of lemon juice
Lightly beat the eggwhite and lemon juice in a bowl with an electric mixer, then gradually add sifted icing sugar, mixing well with each addition. When soft peaks start to form in the icing it is ready to use. Note: Royal icing begins to set as soon as it is exposed to air so keep the icing covered with plastic wrap while you are not using it.
Once you have made the icing, you can divide the mixture into different bowls and colour it with edible food colours. I like to use light colours, so I only add a tiny bit of colouring at a time with a toothpick, until I have the colour I desire.
Next, fill a piping bag with some icing, then pipe a line around the edge of the cookie. I have used piping tip #3 for these cookies.
Take a batch of coloured icing, and water it down slightly so it has a runny consistency. Fill the cookie with the icing, being careful not to let the icing overflow around the edges. Sometimes air bubbles appear in the icing, so have a pin or toothpick on hand to pop them.
While the icing is still wet, you may choose to pipe on different coloured icing. Patterns can be created by swirling the colours together with a toothpick.
Leave the cookies to dry for at least 4 hours, then store in an airtight container.