A grinder is a wonderful aid for that part of the job, but it isn't necessary. The more often you turn the pile, the sooner you'll have finished compost. If you like, you can sift the pile before using the compost or just use the smaller particles you find on the bottom and throw the big chunks back to decompose further. Don't add meat or dairy products to the pile or manure from meat-eating animals. Finished compost can be used as potting soil, as a top-dressing for container plants or as an amendment for existing planting beds.
Water goes right through the mat.
You can place a layer of loose mulch over the mats to make them look better, if you choose. Mats come in different sizes to fit all trees.