One way to do it would be ceramic tiles, you take a dremel and engrave the letters in the tile. when grouting, grout fills the engraved area and it shows up.

I have a Taurus ring saw and I cut out letters and numbers from stained glass with it.

Oh wow!
Dremels?
Taurus ring saws?
You two have totally lost me lol.
Sounds a bit sci fi to me! Don't forget I live in Orstralia. We're a bit behind you lot!
I think I asked once before what a Dremel was and that was explained but a Taurus ring saw? My eldest son is a Taurus. He's a carpenter. He has ring cause he's married. And he has a saw. Well several actually. Would he know?
Actually I think I'll just get some small tiles and use ceramic paint. Sounds easier than messing with Dremels and Taureans lol
Cheers
Wanderer1

I was going to suggest paint that you bake in the oven, like pebeo porcelaine. I don't have a ring saw either.
Kathy

a ring saw is a glass saw. The blade is a ring that rotates.
You might possibly find a potter or someone with a kiln to make you what you want.
Many people also use wood scrabble tiles. or the square beads with alphabet letters printed on them.
Or, small clear glass squares with the letters printed on paper and decopaged to the underside. Or painted by hand onto the underside of clear glass squares.

Here is a link that might be useful: ring saw