Once finished laying the stones upside down, I sprinkle them with a thin layer of grout and spray them with water. After a few hours I follow up with a coat of thinset. I like to mix the thinset with an acrylic additive, to improve its resilience and adhesive power.
This is a small windrose tabletop ready to be filled with thinset. You can see the difference in thickness of the different materials. the black granite is about 5/16" (8 mm) thick, while the red glass is about 1/8" (3 mm) thick. The direct method would not allow such disparity in thickness.
I finally cover the thinset with a structural material, - I like commercial concrete backing boards used to cover bathroom walls to be laid with ceramic tiles.
I let it dry for at least 48 hours before flipping the piece. This is important. I have damaged some pieces for I was to eager to see them. The thinset needs time to solidify and release moisture. The does not really depends on temperature. It simply needs time.
I wrote somewhere else that the flipping of a mosaic is a very exciting thing, like a birth. In a few seconds, you see what you created. It is very different from the direct way, where you set the stones one by one and see your work appear little by little. Here, it is more like a birth; suddenly, the butterfly emerges from its chrysalis.
Here is the Olive Mirror right after being flipped.
Now we are almost done; in "Making of a Mosaic - Episode III", we will complete our work.