The Icing I have found for the best flavor and the ease to work with is:
Buttercream Icing:
For the Signature Thomas or Princess Castle I would recommend doubling the recipe so you are sure to have enough icing, then going by your decorating guide divide the icing and color it according to how much you will need to complete the decorations.
Cream butter and shortening with electric mixer. Add vanilla. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Add milk and beat at medium speed until light and fluffy. Keep icing covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Refrigerated in an airtight container, this icing can be stored 2 weeks. Rewhip before using. YIELD: 3 cups *Substitute all-vegetable shortening and 1/2 teaspoon Wilton No-Color Butter Flavor for pure white icing and stiffer consistency.
**Add 2 tablespoons light corn syrup per recipe to thin for icing cake.
OR:
Try the different icing recipes at Wilton, plus they also have cake decorating tips and hints...
Recipes...http://www.wilton.com/recipes/browse.cfm?category_id=16
Cake Decorating Hints and Tips...http://www.wilton.com/decorating/basic/index.cfm
Using the Decorating Tips, Decorating Bags and the Coupler:
Using a two-part device called a coupler it lets you interchange several decorating tips without changing the bag. The two parts of the coupler are the base and the ring. The base fits inside the bag, then you put the decorating tip of your choice over the portion of the coupler that sticks out of the bag. When you screw the ring on, you've locked the decorating tip onto the coupler and bag.
For the larger decorating tips in the Thomas Cake Kit 2B and 1D first put the tip inside the decorating bag, then snip the end of the bag with scissors until the tip will fit about 2/3 the way in the opening. You do not want the tip to be too close to the opening of the bag so it will fall through. The you fill with icing.
For the smaller decorating tips 5, 8 and 10 follow the instructions below...
Placing Coupler onto the Decorating Bag:Unscrew the coupler ring. Notice that the threads in the coupler base start about 1/2 in. above the tip. Push the coupler base into the bag as far down as possible. With scissors, trim bag about 1/4 in. below bottom edge of coupler. Position decorating tip over coupler base and bag. Screw ring in place to secure. To change decorating tips, unscrew ring, replace decorating tip and screw on ring.
Filling The Bag:The most important thing to remember is don't overfill, or icing may squeeze out the wrong end. The right amount of icing to start with is about 1/2 cup. Fold down the top to form a generous cuff, and hold the bag beneath the cuff. Use your spatula to fill the bag with approximately three tablespoons of icing at a time. To remove icing from the spatula, hold the bag on the outside between the thumb and fingers. Then pull the spatula out of the bag, using the bag and your fingers to squeeze the icing off.
Closing the Bag:Unfold the cuff and twist the bag closed, forcing the icing down into the bag. You can make sure you've released any air trapped in the bag by squeezing some of the icing out of tip into the bowl. This is called "burping" the bag.
Holding The Bag:To hold the Featherweight and Disposable bag, place the twist in the V between your thumb and forefinger. Remember that it's important to be able to apply pressure with all your hand. Fatigue in any one finger may signal a problem in your grip. While you're decorating, it will help if you steady the tip of the bag with one or two fingers of your other hand. This helps support the weight of the bag.
Making Flowers and Decorations with the Tips:For tips on how to form the flowers and other decorations with the tips go online to the Wilton Cake Decorating Site... www.wilton.com/decorating/basic