Layer and fold a few sheets of heavy duty tin foil. You may need to tear off sheets in both directions depending on the size of your slow cooker. Gently press your tinfoil into every edge of your empty slow cooker so the tinfoil is completely covering the inside. Make sure to fold the ends of the tinfoil over the top edge of the crockpot where the lid will sit. It doesn't have to be perfect. The food will add weight to help to press the tinfoil down as well. Another option is to buy Reynolds Kitchens Premium Slow Cooker Liners  each measuring 13 x 21 inches to fit 3 to 8-quart round or oval slow cookers if you'd rather not make a homemade version yourself.