Be sure to use thinset mortar that is latex- or polymer-fortified; otherwise, it is likely to crack, resulting in loose tiles. You can buy “polymer-fortified” thinset mortar that you mix with water, or you can buy less expensive, unfortified thinset and add liquid latex as directed.
Use gray thinset for ceramic and stone tiles that are dark in color. However, many types of marble, glass, and other kinds of tiles are slightly translucent, so a gray mortar would slightly muddy them. For these tiles, use white thinset, which costs only a bit more.
Mix the thinset mortar and allow it to rest the amount of time specified by the manufacturer. Slowly add a couple of inches of liquid to the bottom of a 5-gallon bucket and then pour in some powder (don’t add water too quickly). Mixing by hand can be somewhat strenuous work-a half-inch drill fitted with a mixing paddle makes the job much easier. Hold the bucket between your feet as you spin the paddle. Work slowly at first to avoid slopping mortar out of the bucket. Then run the paddle faster until the mortar is a smooth consistency. When you pick the paddle up, the mortar should remain stuck for a few seconds before sliding off. Keep mixing until you attain a consistency that holds its shape and is barely pourable.
Starting in a corner, dump or scoop the mortar onto the surface. Use the flat slide of the trowel to spread the mortar over an area about 3 feet square. Using the notched side of the trowel, comb the mortar to produce an even surface. Use long strokes and hold the trowel at a consistent angle. If the mortar starts to harden while you work-either on the surface or in the bucket-discard it and mix a new batch.