Park in country land and follow public access route as shown on O/S map. Stone is on the right in second field you come to. Easy to spot - large flat stone in middle of field. Lots of cup marks on surface. When I visited I found a lucky horse shoe!
A note by J Wiles in the National Monument Record (NMR), available via Coflein, describes Maen Catwg as,
A roughly rectangular prostrate stone, 2.6m by 1.7m and 0.6m thick. There are 33-50 cup-marks on the upper surface of the stone, which has been suggested to have been the capstone of a collapsed megalithic chamber.
The NMR also describes a second cupmarked stone discovered in 1990 by T Driver (him again) in a pile of cleared boulders in a field near Maen Cattwg. The record states that, "the present status of the second carved stone is not known". I think that means they lost it.
The eight figure grid reference given for Maen Catwg is ST12709742 and the National Primary Record Number (NPRN) is 93097.