Arrange your tile pieces and marbles on your mosaic surface in whatever pattern you'd like. Play around with your designs until you're satisfied, and then take a picture to use as a reference for the actual process.
Using your paintbrush or foam brush, apply a thin layer of Mod Podge to your coaster to seal the surface. Allow it to dry thoroughly before moving to the next step.
Using your paintbrush, apply a thin layer of glue to a small part of the coaster. Only apply it to one area at a time, or it will dry before you can finish.
Begin applying your tiles to the glued area. If you need to, use the picture you took at the beginning as a reference. Do not leave more than 1/4" in between tiles, though I prefer about an 1/8". Once you've covered the glued area, continue applying more glue and tiles until you've finished the entire coaster. Allow this to dry at least 24 hours before moving on to the next step.
Pour some powdered grout into a small bowl, and slowly add water, stirring constantly, until you have a thick but smooth consistency. The consistency should be similar to thick pudding or peanut butter. Try not to add too much water, but if you do, just add some more grout to thicken it back up.
Using a popsicle stick or an old credit card, spread a thick layer of grout onto your tiled coaster. Spread it all over the surface, using enough to fill in all the cracks. The grout will stain, so do this over a protected work surface, and wear gloves if you don't want your skin dyed.
Using your grout float or the piece of dense foam pool noodle, squish the grout into all the cracks and crevices, paying special attention to the uneven areas, like those around the glass beads. Let the grout cure for about ten minutes before moving on to the next step.
Scrape away the excess grout with your old credit card, and then using a lightly dampened sponge, begin to wipe away the rest of the grout. Be careful that your sponge is just damp because too much water will wipe away the grout between the cracks as well. Leave your freshly-grouted tiles to dry for several days before sealing.
If you'd like, paint the sides of the coaster. Now, dip a soft cloth into the grout sealant, and wipe it onto the entire surface of the tile, buffing away any leftover grout dust as you go.
You can make different patterns and experiment with different findings and textures. These make great gifts and are pretty enough to be left out all the time. They sure beat a potholder for protecting your table from fresh-from-the-oven dishes!