When I take my icing out of the mixer, this is what my icing looks like. It is stiff, but will still hold a peak. If you find that the icing cracks when you pipe it, it may be too stiff. In this case, add a couple drops of water. I use this consistency when I am making roses, flowers with petals and borders..
This consistency is achieved by adding a few drops of water to the stiff icing. I generally add the water slowly, as it will thin out quickly. I use this consistency for royal icing transfers, lettering, shell borders and flowers with flat petals.
This is the consistency that is generally used on the cookie surface to provide a smooth and beautiful surface. Start by adding a few drops of water to your stiff icing and mix it up. Continue this process until you get to the desired consistency. The icing usually smooths out within 12-14 seconds. It should not run off the sides of the cookie. If it does, it is too thin and you can simply add some more confectioners sugar to stiffen it up. Eventually, you may be able to gauge the consistency simply by eye. It will take time, but you will master it!