How do I use these? Do I use before or after fondant is on the cake?  Makes sense to apply before covering the cake however, what if I only want the impression on the sides of the cake --brick work and not on the top?

AYou can use the rolling pin before the fondant is on the cake, the mat you can use before or after. When the cake is covered with fondant press the mat on to the surfaces you want to imprint, there are a lot of YouTube tutorials you can watch :-)

Thank You going to YouTube right now.