I get a lot of questions on how I decorate my cookies with royal icing. I like the method where you first outline the cookie and when the outlining is dry you flood the cookie with a runny icing. It gives the cookie a smooth look and when its dry, you can pipe details on the cookie. Flooding cookies is easy and so much fun to do.
I use a piping bag with soft royal icing and a tip 3 for my outlining most of the time. I make my royal icing soft with few drops of water, but not softer than it keeps its shape when I pipe. Stiff icing will make the icing curl and it can be hard work for your hands. Try and pipe a few strings on your worktop, to see if its good consistency.
I hold my bag about 1/2″ above the cookie. That way I can navigate the icing without breaking it. It takes a few practice. For the second outline on the cookie I use a piping bag and a tip 2 with soft icing. If you are not happy with the outlining, use a small spatular to scrape off the icing and start over.
Full Post: How to flood cookies with royal icing