Meringue powder is an icing mix available in cake-decorating supply stores. This recipe is a good choice if you're concerned about consuming raw eggs.
Place meringue powder and 3/4 cup water in the bowl of an electric mixer and use the whisk attachment to combine. Add 1/4 cup of the sugar; mix well on low speed. Gradually add remaining 3 3/4 cups sugar, mixing constantly and scraping down the sides.
Increase the speed to high and continue to beat until soft peaks form, about 5 minutes. Gradually add more water as necessary to thin to the consistency appropriate for the decorating you are doing. As a rule, the icing should be stiffer for lettering, more malleable for making petals -- but you will need to experiment to find the best consistency. Store at room temperature in an airtight metal or glass container for up to 2 weeks.