This is the icing recipe I use for my decorated cookies. I've used it since I first started doing the cookie bouquets three years ago. I know several people who use the rolled buttercream and the rolled fondant, and I've used both of those as well, but I always come back to this recipe and technique.
1 lb. powered sugar ( a little shy of 4 cups)
1/4 C + 2T whole milk (I usually use 2% since it's what I have)
1/4 C + 2T light corn syrup
1 tsp vanilla (or almond extract - I use the same as I used in the cookies)
Combine sugar and milk with whisk until smooth. Be sure to get all the lumps out. Stir in corn syrup and extract.
I use this same recipe for the glazing and the piping. For the piping, take a small amount and add powered sugar to make it thicker. If it gets too thick, just add a few drops of milk.
I figure out what colors I'm going to need for my cookies. I then divide the glaze up in small bowls and mix in gel food coloring.
For the Valentine's Day Cookies and the St. Patrick's Day cookies, I outlined the cookies with the thicker glaze, then filled in with the colors I wanted.
For the previous cookies and cookie bouquets, I put waxed paper under the cooling racks and poured the glaze over the cookies. I then piped the thicker glaze on them.
I'm not sure which way I prefer - still trying to figure that out!
On the V-Day and St. P-Day cookies, I used a toothpick to make the designs. Just drop a dot of a contrasting color onto the just-glazed cookie and pull a toothpick through the dot to make the designs.