Recipe modified slightly from Sweetopia.
If you're going to invest in one thing when making royal icing, make it your meringue powder. Flavor and texture is important, so don't settle for the powder you find in most craft stores.
I assemble my piping bags while the icing is mixing to save time. Always use a coupler when decorating cookies -- it makes it so much simpler to change out tips during the icing process.
I generally use one icing consistency on my cookies, but you definitely can add more water (slowly -- just a teaspoon at a time!) if you want a thinner consistency for flooding. This can speed up the decorating process.
If you're tinting your royal icing with food gel, remember to only add just a bit at a time. The color is more potent than one may think. I recommend adding just a small pin-size drop at a time until you're familiar with the color and its strength.

FAVORITE ROYAL ICING

INGREDIENTS

3/4 cup warm water (plus more for thinning icing consistency)
5 tablespoons meringue powder
1 teaspoon cream of tartar
2 lbs. powdered sugar
Optional: 1/2 teaspoon extract of your choice (I always use vanilla, but almond and lemon are also popular choices. Just avoid anything oil-based, and if you want pure white icing, use clear extract)
1 teaspoon light corn syrup
Optional: gel food coloring in shade(s) of your choice

In mixer bowl, combine water and meringue powder; hand whisk for 30 seconds.
Add cream of tartar; hand whisk for 30 seconds.
(Optional) Add extract.
(Optional) Add light corn syrup.
Add all of the powdered sugar; mix on low speed for 10 min (use "stir" or "2" on most stand mixers).
Stir in small increments (1 teaspoon at a time) of water until you reach a "10-second" consistency, which means when you run the tip of a knife through the icing, the line disappears in 10-seconds. This will result in an ideal consistency for outlining and filling the cookies.
(Optional) Tint with food gel (I swear by Americolor).
Keep royal icing covered with plastic wrap at all times. Store with a damp cloth and plate (same diameter as top of bowl) on top in bowl in refrigerator for up to 3 days.