What do you do with tiles of different thicknesses?
It happens all the time: your main shower tile is 3/8" thick or thicker, and your glass accent tile or mosaic is 5/16" or less, with some as thin as 1/8". Combine that with the fact that larger tiles need more thinset, which will give them even more height, and you could have a height difference of almost 1/4"!
Use a slice of 1/8" building material as an easy way to add height behind a thin mosaic or tile so it matches up to the surrounding material.
In the above cross section, you can see how the larger trowel required for the thicker, larger tile adds 1/4" of height. To match this, we added our 1/8" board with a layer of thinset above and below. A thinner bed of mortar under the small mosaic tiles prevents it from oozing up and getting into the grout lines.
NOTE: After troweling your thinset for your mosaic, use the flat side of your trowel to knock down the ridges for smooth surface. You're not removing any thinset, just flattening it to get an even surface. This will prevent thinset from oozing up between your tiles and creating a mess.
Using this technique, you can achieve smooth, flawless floors, backsplashes, and walls, as seen below.
Wedi recommends this same technique with their 1/8" and 1/4" board, as seen in this video where they have pre-set and pre-grouted their mosaics: