Flip the mat over so any curled portions are facing the floor.
Set books, pans or other heavy items over any unruly portions of the mat, pressing the material flat. Leave the weighted items in place for 24 to 48 hours.
Remove the weights and use a measuring tape or ruler to establish both the length and width of the mat.
Add 1 inch to each dimension and use the adjusted measurement to determine how much slip-resistant rubber padding is needed. For example, if the doormat measures 24 inches long and 14 inches wide, the padding will need to be at least 25 inches long and 15 inches wide.
Unroll a piece of highly textured rubber carpet padding; if synthetic carpet backing is not readily available, a sturdy, vinyl bathmat makes an acceptable substitute.
Cut the padding to the required dimensions with a utility knife or box cutter.
Brush a thin layer of all-purpose plastic adhesive around the edges of the mat. Place the padding in the center of the backside of the doormat and press firmly to seal the adhesive.
Turn the mat right side up and set it aside. Let the glue cure for 24 hours, then use the utility knife or box cutter to trim the excess padding from the edges of the doormat. Return the mat to its usual position.