Some basic knowledge of how the "right" consistency of any kind of icing feels has to be learned by trial and error (or in a PME class). Some of us got that at our mothers knees and other joints.
Royal icing consistency is easy to control when you make it the British way.
Start with 3 "large" egg whites and a pound of powdered sugar (have some extra from a second pound handy). Let the egg whites come to room temperature after you have separated the yolks. Add a 1/4 teaspoon cream of tartar (powder) and beat on medium speed. Add the whole of the first pound of sugar slowly and keep beating until the icing can hold a soft peak when you pull up a spot with a table knife. It should take 15 minutes of beating, DO NOT whip at high speed.
That is justabout your ideal consistency for borders. Add a little more sugar for flowers, how much will depend on what kind. Soften for stringwork and leaves by adding corn syrup.
Then the only way to get better is to pipe for an hour every day. Fill up an icing bag and practise practise practise. If you use a coupler then you can make some borders and some flowers from the icing. It is best to use the Wilton icing that has shortening sugar and water, because it can be scraped off and re-used.