Sometimes, it is easier to mosaic on mesh first, rather than directly onto the substrate.
I buy my mesh from a Tile shop. Then, once I have my design idea drawn onto the mesh, I apply a watered down glue to the back of the tile and stick down on the mesh. Except it’s not that easy. All sorts of things can go wrong. The glue might block the holes of the mesh, the mesh gets stuck to your work surface. If the weather is wet the glue never seems to dry. If the completed mesh squares get wet, or are kept in a humid environment, the glue stops working and the pieces fall off. All of these things have happened to me, and were accompanied by lots of swearing and worrying about the commission being a big flop.
Then I would go to sleep and in the morning I would have the solution. Use a wet towel under the mesh to prevent the glue problems. Carefully drag the mesh squares onto a board and transfer onto metal drying shelves- before the glue dries. Use a sprayer to keep the towel wet while you work. Dry the completed work thoroughly and store away carefully in a dry environment.
Installation is fairly straightforward, place your mosaic mesh squares onto a prepared bed of tile adhesive. Tap down using a flat piece of wood until the adhesive oozes up through the mesh. Allow to firm a bit, sponge off, and clean up, apply grout the following day.
I always use bonding liquid in the tile adhesive and grout, which makes the mosaic strong and waterproof. I would never mosaic on a timber substrate, (because unless the piece lives in a controlled indoor environment, it wont last)