From Anne's Food

for outlining: 

250 ml (1 cup) powdered sugar

2 tsp meringue powder

2 tbsp warm water

1/2 tbsp lemom juice or 1/2 tsp vanilla extract

for flooding:  

250 ml (1 cup) powdered sugar

2 tsp meringue powder

4 tbsp warm water

1 tbsp lemom juice or 1 tsp vanilla extract

Mix all ingredients and beat on high effect for about five minutes. It will be thick and shiny. Add color, and if it seems too thick, a few drops of water. Move to a piping bag and seal it immediately, or cover with plastic wrap - it will dry out very fast. 

If you want to flood, you have to start with outlining. Outline, and let it dry slightly. Then flood. It'll take about 24 hours to dry - after that, you can decorate using a different color.