With large litter mats like this, you simply need to place your litter box or boxes on top. It is best to measure your cat's litter box to be sure you purchase the right size as some boxes are larger than others.
Remember, in order for mats to work, your cat must step on it. This allows any stray litter to hopefully be dislodged and keep it from spreading to your floors or carpets.
One of the benefits of purchasing a Petco litter box mat like this, is that your cat's paws will not be irritated by hard ridges or bumps.
Some litter mats made of plastics have these "ridges" built in, which may cause some aversion to the mat.
With any luck, your cat will be stepping easily on this soft, flat Petco mat.