need help with bytter cream preparation for decorating/icing birthday cake

hi,
can please someone help me. i need to decorate a cake for my daughter's birthday next week on the 25. i tried preparing butter cream icing using following ingredients...3 cups icing sugar, 3 tbsps unsalted soft butter,2 tsps vanilla , 2 tbsps milk. initially the cream was nice. but once i started piping it, it started to give a crystally texture appearance. i need to use this for covering and decorating my cake, so can somebody give a good recipe for a smooth cream please????

where did i go wrong?? i saw on the Wilton website something about adding shortening??

and i have bought whipping cream..it is from a brand called merry whip, can i use this for decorating and covering my cake?? and after decorating do i need to put the cake in the fridge ?? both for butter cream as well as whipped versions????

can somebody please help me???

looking forward to your replies...
thanks
shubha