All of these have been inlaid with grout, something just a little different and it's fairly quick and easy to do.
Just press the grout into the letters with a putty knife then scrape it of level the wider the putty knife the better, then with a stiff damp sponge clean the stone up ,let dry then seal it after a couple stones you will be a pro. White and Charcoal colors have worked the best for me but you can use any color you like. I have also done this after painting and on stained stones, so be brave and give it a shot.
Acid stain was bought at Direct Colors online, you will also need to use a sealer.Hope to add a few videos soon.