We have done it several different ways. You can buy the adhesive rubber mat and cut your own stencils. But it is much easier if your can get the Monument company to use their computerized mat cutter to cut the mat for you and pay them for that.
We have placed information marker in this way at Sulphur Springs, Marks Mill Cemetery, Mt Elba, Longview, Lee's Ferry and other sites in this manner using discarded stones from the Monument companies junk pile, resurfacing them and sandblasting the new engraving into them.