Pour the thinset into a plastic grout bag until the bag is half full. A 1-gallon zip lock baggie works well for a grout bag.
Cut the very tip of the bag’s corner off with scissors. If you have a trout bag, the corner will have markings to show you where to cut. If you are grouting a standard ¼-inch grout line, cut approximately 1/8-inch off the tip of the bag.
Squeeze the thinset out of the corner of the bag and into the grout joint. Fill joints until the bag is empty.
Wipe excess thinset off the tie with a wet sponge.
Radius the grout with the tip of a dowel rod or by running the tip of your finger over the surface of the grout. Again, remove any excess thinset from the surface of the tile with a wet sponge.