1. Bake cookies 1 day ahead.
2. Mix royal icing, divide into small portion, add color.
Icing consistency is important but doable. 10 second rule is good to start-it is the time for icing smooth itself naturally after being drop back to the icing bowl but 12-15 seconds is good. Try the 10 seconds first and add a little bit sugar more to make 12-15 seconds to see which one work best for you.
Draw the line of color area you want to do with a food writer pen or if confident, you can just mark it out mentally. Need to outline first with piping icing around area first. I just use 10-second icing for both piping (drawing lines) and flooding (filling large area). Then flooding with the same color or different color. If you want the outline blending with the filling-you should fill in right away and make sure the edge of filling touching the piping. If you wait a few minutes between, the filling and piping will not blend.
The tied dyed color is using technique “Wet on wet” Piping and flooding one color and immediately add another color next to it but work from the far ends first and reach to the joint area. When both colors touch each other, use a toothpick and drag it from one color to another. Wipe the toothpick and repeat until finish your design. You can fill a large area with one color and pipe a line of different color atop of it then use a toothpick to drag these different color icings together.
The heart shape is made by dropping icing immediately on a wet filling surface and use toothpick to draw the shape. You may make several circle of different color icing at one spot and make a multiple color heart too.
The icing will stay on the top if adding after the filling dried a little bit or overnight.