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.
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.
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!