Make a batch of royal icing and color it as you wish.
Scoop the icing into a piping bag and using a small round tip, pipe dots of different sizes onto a cookie sheet lined with parchment paper.
Leave them overnight to harden and then carefully pick them up with a thin spatula and place them wherever you like on your cake.
If you are piping letters or numbers it would be helpful to print out the letters on a sheet of paper and slide it under the parchment sheet.. it will serve as a guide for you to follow with your piping bag.
Pictures and directions found at: