I have done this on fondant and made a template out of card stock and traced it onto the the cake. You need to figure out the spacing, so it works out evenly. The template made it so much easier.
Hope that helps.
You need to place the fondant on the cake and then press the mat into the sides of the cake, if you have a silpat place your cake on that dust your impression mat with corntsarch or luster dust(remove the excess) and press the mat into...
I have used mine with sure cuts a lot(can't do that now). Maybe once every few months ,but I also use mine for paper, I have liked it when I needed to use it, for small things I still use my exacto knife.