Substrates need to be clean, stable, plumb & true, and also free of dirt, stains and solvents. Cement or cement board are preferred. New cement will be 90% stable within ten days of pouring, but further shrinkage and crystallization will continue, New cement substrates can be tiled over after ten days, if the substrate is first covered with a crack suppression membrane.
Do not install glass tiles directly over plywood or drywall.