I use ready made frosting all the time but I put the frosting in my mixer and whip it up and it spreads very easily.
I usually use about 2/3 of a container for a 2 layer, 9 inch cake. Most of the time I use the chocolate flavor type.
Store the left over frosting in the fridge and use it for cup cakes.
Grands love the cup cakes better than the layer cake...