I just did my first real cake deocrating today. I bake all the time but this was my first themed cake. I decided to use the Wiltons BC icing mix for the spread. The directions said if you are spreading it, add at least 1 extra TBSP of milk. I did just one. Well, when it came time to spread it, it was way runny. Did one TBsp make that big of a difference or is it crappy icing? I used precolored fondant for stars, circles, and mickey heads. That part came out great. The icin looks like crap, though. I am thinking next time just to use white fondant.......i was just nervous about maybe coloring the icing and not knowning how to color fondant. I didn't color the BC.

Welcome to Cake Central. As with any icing, it can often require adjustment, powdered sugar if it is too thin, some liquid if it is too thick.

Depending on the humidity and other factors, the same recipe may not require the same adjustment every time.

Our Wilton instructor advised us to always add liquid 1 tsp at a time, even if it calls for 1 full tablespoon...

I had the same trouble so I just started making my own from a recipe on here and it is fabulous it is well worth the effort to make it...

<...Did one TBsp make that big of a difference or is it crappy icing? ...>

BOTH!!
As others have said It is soooooo easy to make b'cream you don't need to use a mix.