I will also share my no fail sugar cookie icing recipe.
Just a drop at a time. If you have already outlined your cookies and need a to fill in or ’spill’ your cookies, you can add more milk (as much as you would need) to make your frosting very runny.
If you are using food coloring, be sure to use LESS milk. Most food colorings are liquid and can make your frosting more runny. If you are using a gel food coloring, your milk quantity can remain the same.
If you want to add different flavors, you can add drops of vanilla extract, almond extract, lemon, etc. to flavor your frosting.
If you want more SHINE, use more corn syrup and less milk. This frosting will stay good for days in the fridge.