My experience with silkscreen stencils for cookies is that I like to make my icing consistency thinner than what I make for my plastic stencils. Now with that said, you can make a thinner consistency for your plastic stencils, although my personal preference for those kind of stencils is to the have a stiff dramatic effect. And a stiffer consistency will not be your friend for silkscreen stencils.
So, first things first.... Let's make the basic royal icing recipe HERE.
And then take a few tablespoons into a smaller bowl and thin it down to a ten second consistency.