Do you wait for the cake to be cold before covering it with ready made icing?

Yes, your cake should be thoroughly cooled before you ice it, whether the icing is homemade or readymade.
Janice

Unless it's that new icing you pour right on the hot cake. But it would be clearly labled as such.

..lotsoftots...I just saw that the other day! It is also printed on the side of the cake mix box that you can now frost a warm cake with that new icing! Never tried it though!