Ceramic tile is a rigid product that has a high compressive strength. This means you can squeeze it and it won't crush. However, ceramic tile, concrete and natural stone have low tensile strength.
This means if you try to bend a piece of tile, it cracks or snaps fairly easily.
1. Avert tile cracks
To prevent cracked floor tile, you need to make sure there are no spaces between the tile and the floor surface it rests on. A space under a tile will cause the tile to bend as you apply pressure to the top. The larger the tile you're installing, the more difficult this task becomes, as it's very hard to get a floor surface in the same plane.
If you plan ahead, you can use a pourable self-leveling floor compound to get a perfectly flat floor surface.
Cracks in a concrete slab can, and often do, telegraph through the floor tile. If your slab has cracks, consider using special injectable concrete epoxies to repair the cracks so the concrete doesn't move.
Professionals often install crack-isolation membranes between the tile and concrete slabs. These help keep ceramic tile crack-free for the life of the floor.
2. Spacers vs. a grid
Plastic spacers used to keep grout lines straight don't always work as intended. It's not because the spacers are defective. Realize the spacers are probably manufactured with very tight tolerances, but the ceramic tile may be off slightly. If you have a variance of just 1/32 inch between tiles, that can create a 1/16-inch difference if two of those tiles are installed side-by-side. Pretty soon, the grout lines will look more like a sinuous stream channel if you use the plastic spacers.
You might try to create a grid on the floor using a chalk line. Calculate the exact distance, based on the width of the tile and the grout line spacing for either a 3-by-3-foot tile grid or possibly a 4-by-4-foot tile grid. Snap the lines, making sure they're accurate, and lay the tile to the edges of the grid lines. The center tiles are just installed by sight, making sure the grout lines are consistent and straight.
3. Grout's magic
Before the grout is installed, the lines between the tiles may appear less than perfect. Grout has the magical ability to mask very slight imperfections. But if you have twisted or very crooked tiles, it will still look twisted and crooked with grout.
When you get ready to grout, you can spoil all your hard work up to this point. Rookie tile setters often make any number of mistakes when mixing, applying or finishing the grout. Purchase some very inexpensive floor tiles and install them on a 3-by-5-foot piece of cement backer board. Practice grouting on this test area until you perfect your skills.
4. Be consistent
Mix the grout so it's the consistency of bricklayer's mortar. If it flows from a bucket like pancake batter, it's too wet. Too much water weakens the grout. This will cause it to crack or crumble in the months ahead.
Apply the grout with a hard rubber float at a 30- or 45-degree angle to the grout lines. This diagonal stroke will ensure the grout is flush with the top of the tile. Do not rub the grout with a sponge until it firms up enough so you don't remove grout from the joints.
5. Use small batches of grout
Low humidity and hot temperatures will cause floor grout to set up rapidly, making it hard to finish. You need to discover on your own how much grout you can install and finish at one time. Mix only enough grout in a bucket that you can install and finish in one hour. You can always mix more.
Don't add water to stiff grout in the bucket. Retempering the grout with water will weaken it.