Use medium consistency royal icing and a decorating bag fitted with round tip 3. Hold the bag at a slight angle (45°) with the back of the bag facing towards you and touch the tip to the surface.
Squeezing the bag with steady pressure, draw the icing on the surface if piping on a vertical surface (pre-assembled house). If piping on a flat surface (unassembled house), lift the tip slightly after attaching the icing to the surface and let the icing drop into place, guiding the placement of the line as the icing is squeezed out of the tip.
To end the line, stop squeezing and pull the tip away.
Fill in the outlined area using a zigzag motion.
Immediately pat smooth using your finger dipped in cornstarch.