Apply the Tiles to the Mastic
Before pressing the tile into the mastic, go back over the whole area with the flat edge of the trowel to knock down the ridges (Image 1). This will prevent the ridges from showing through the glass. Even after they're knocked down, though, the ridges create air pockets that help adhere the tiles.
Set sheets of tiles into the mastic according to the layout (Image 2). Apply firm, even pressure to each tile to make sure it's set in the adhesive. If tiles fall off the sheet, simply press them into the adhesive; the beveled tile edge provides the spacing needed.