If the consistency of your icing is not right, your decorations will not be right either. Just a few drops of liquid can make a great deal of difference in your decorating results. Many factors can affect your icing consistency, such as humidity, temperature, ingredients and equipment. You may need to try using different icing consistencies when decorating to determine what works for you.
As a general guideline, if you are having trouble creating the decorations you want and you feel your icing is too thin, add a little more confectioners’ sugar; if you feel your icing is too thick, add a little more liquid.
In royal icing recipes, if adding more than 1/2 cup confectioners’ sugar to thicken icing, also add 1-2 additional teaspoons of Meringue Powder.
Options
option 1: stiff icing
Stiff icing is used for decorations such as flowers with upright petals, like roses, carnations and sweet peas. Stiff icing also creates your figure piping and stringwork. If icing is not stiff enough, flower petals will droop. If icing cracks when piped out, icing is probably too stiff. Add light corn syrup to icing used for stringwork to give strings greater elasticity so they will not break.
option 2: medium icing
Medium icing is used for decorations such as stars, borders and flowers with flat petals. If the icing is too stiff or too thin, you will not get the uniformity that characterizes these decorations. Medium to thin icing is used for icing your cake. Add water or milk to your icing recipe to achieve the correct consistency.
option 3: thin icing
Thin icing is used for decorations such as printing and writing, vines and leaves. Leaves will be pointier, vines will not break and writing will flow easily if you add 1-2 teaspoons light corn syrup to each cup of icing.
Source:wilton