Here is the Vajra Guru Mantra, nicely chiseled in a nice rock, up behind Tsodzong Gongba (Lake Castle Monastery -- link here, but take it with a grain of salt). This is on a tiny island in the middle of East Tibet's largest lake, and has long been associated with Dudjom Rinpoche and his family.
It isn't all that difficult to carve mantras in rock. For a beginner, all you need is patience and the proper tools. Above is a common set, available from most art supply stores. You begin by laying out the design in chalk, then use the tools to cut away from the edges, bringing the design into relief. You can use the above example of the Vajra Guru Mantra for your first attempt: it is a model of correct work -- some speculate it might have been done by Dudjom Rinpoche himself!