After setting floor tiles in mortar, wait at least overnight for the mortar to harden. When it has turned a lighter color, this indicates that it is really dry.

Use sanded grout for joints that are 1/8-inch wide or wider and unsanded grout for narrower joints. The grout must be fortified, either with a “polymer” that is incorporated into the powder, or with a liquid latex additive that you add while mixing. Follow the manufacturer’s instructions.

1) Prep the surface. Wipe the surface with a damp cloth or sponge, cleaning away any residual mortar. In the joints, the mortar should be at least 3/16 inch below the tile surface; otherwise it may show through the grout. Use a knife, screwdriver, or putty knife to carefully dig away any mortar that is too high. Clean away any mortar crumbs.

2) Mix the grout. Pour a bit of latex additive or water (depending on the label instructions) into a bucket. Add grout powder and mix, using a margin trowel. Slowly add the liquid and keep mixing, taking care not to add too much. Mix until you achieve grout that is smooth and about as thick as mayonnaise. Wait 10 minutes for the grout to “slake,” or become fully dissolved, and then mix again.

3) Fill the joints. Pour or scoop some grout onto the floor. Holding the grout float flat, run it over the surface in several directions, pressing the grout firmly into the joints.

4) Squeegee. Turn the float to about a 45-degree angle and scrape away most of the grout on the surface. Work diagonally to the joints to avoid digging the float into the joints. Use medium pressure. Aim to squeegee away most of the excess grout, but don’t linger and try to be perfect; you will clean away any residual grout from the surface later but before the grout dries.