2. Glue the mosaic to the tile. You can use just about anything in your mosaic–broken glass or pottery, bottle caps, stones, jewelry, coins, etc. Since we’ll be using these as coasters, however, the only thing to watch out for is that your mosaic pieces have flat tops. My kids love to make mosaics using marbles and game pieces, but those wouldn’t provide a stable surface for a drink, so we went with broken colored glass here. School glue or any other plain white glue is fine to glue the mosaic onto the tile. Glue your pieces down, then let the glue dry completely.
3. Grout the mosaic. Mix up some grout, following the directions on the package. Then, using a popsicle stick or your gloved hand, spread grout evenly over your mosaic. The goal is to fill all the grooves between pieces, and keep the surface somewhat level. Give a good edge to the grout around the perimeter of your tile, and cover all the bare surfaces of the tile. Scrape as much excess grout as you can off of the tops of the pieces, but don’t worry that you can’t make them come clean–you’ll take care of that in the next couple of steps.
4. Clean the pieces. Let the tile dry for 10-20 minutes, then grab an old dishtowel, holey sock, stained kid’s T-shirt, or any other piece of soft fabric that’s old enough that you can use it and throw it away without guilt. Dampen the cloth and wring it out very well, then use your finger, wrapped in a section of the cloth, to clean the excess grout off of every mosaic piece on the tile. When you’re done, let the mosaic dry some more.
5. Buff the pieces. After another hour, take the same cloth (which should be dry now), and wipe the mosaic pieces again, this time to polish them. Any last bits of grout left on the pieces should look powdery by this time, and be pretty easy to wipe away. Make the pieces look really good, then set the mosaic away again to cure, this time for at least 24 hours. If you want to seal your grout, you can buy sealant at the hardware store and follow the instructions to seal it, but even outdoors on the patio table under an umbrella, my little mosaic coasters weather well.