Cutting onyx tiles requires specialized tools and skills. Here are the steps to follow:

1. Gather tools: You will need a wet saw with a diamond blade, safety goggles, and a respirator mask.

2. Prepare the tile: Clean the tile and mark the cutting line using a marker or tape.

3. Fill the saws water reservoir: This will keep the blade cool and prevent the onyx from cracking.

4. Wear safety gear: Put on safety goggles and a respirator mask to protect yourself from dust and debris.

5. Turn on the saw: Turn on the wet saw and wait for it to reach full speed.

6. Cut the tile: Slowly guide the tile into the saw blade along the cutting line, making sure to keep your hands clear of the blade.

7. Smooth the edges: Use a wet tile saw or sandpaper to smooth the cut edges of the tile.

8. Clean up: Wipe down the tile and the saw, and dispose of any debris.

Note: Its essential to be careful while cutting onyx tiles as they are delicate and can easily break or crack. If you are not confident in your ability to do this, its best to call a professional.