Bakers usually suggest that you use confectioner's sugar to make frosting stiffer since it contains cornstarch (which is a thickening agent) and the sugar will blend with the icing's sweetness. When you use flour to thicken frosting, the texture will become smoother. However, flour should be used sparingly, since its starchy taste will be noticeable if too much is added to the icing.
Measure 1 tsp. of all-purpose flour and add it to the bowl of icing.
Blend the flour into the icing with a hand-held mixer for three minutes or until the ingredients are completely combined.
Test the frosting's consistency by lifting the mixer's beaters from the bowl. When the icing thickens it will form small peaks. If the frosting appears thick enough, use it to frost the dessert. If it's still too thin, go to Step 4.
Add 1/2 tsp. of flour to the icing and mix them together for another three minutes. Repeat Step 3 and Step 4 until the frosting reaches the right consistency.