To avoid scratching the glass tiles, grout with unsanded grout.
After the mortar sets and the tiles have been cleaned of any excess mortar, mix up the unsanded grout to the consistency recommended by the manufacturer’s directions.
Apply the grout with a float, gliding over the tiles at a 45-degree angle . Don’t apply too much pressure or you might sink the tiles into the thin-set or push them out of plumb.
Back off from your work now and again to check that no tiles have moved and everything is in order.
By the time you get to the end of one wall, the first section of tiles should be dry enough to wet sponge. Wipe the grouted tiles clean with a wet sponge, applied at a 45-degree angle, being careful to keep from indenting the grout lines.
When the grout is dry, polish the haze off the tiles with a soft cloth.