Let's start by talking about the slate coaster.  You can leave it in its natural state, the image will come out a little lighter, but the slate will maintain its porous qualities.  You can also spray it with an acrylic sealer, gloss or matt, or what I've chosen to do, is apply a small amount of mineral oil with a lint free cloth.  For best results, I've found that this step should be done before engraving.  Make sure the tile has had plenty of time to dry.

Bring the image into Creative Space.  resize to 3".  Open the “Adjustments” panel and change the “Sharpness” to 100% and adjust the “Grayscale” back about quarter of the way.

Draw a square that represents the size of the coaster, in this case that is 4".

Place the image into the 4" square and size it to fit.

Next, using the text tool add your text.  Select the text and change the layer color to GREEN.

Click on the image, open the “Adjustments” panel and click the “Invert” box.  Note, you only need to do this if the material you will be engraving on is dark.

Choose “Stone Coaster” for the material.

Click on the “Green” layer, this will select the text.  Select “Engrave” and enter the parameters:

Click on the image, Select “Engrave” and select the parameters.

Place your slate tile on the laser bed, make sure the material is under the red dot, and perform the auto measure. 

Perform the “Framing” function.  When you are happy with the placement go back to the file and delete the square, you no longer need that.

Send the job to the laser for processing.

When complete, remove from the laser and wipe it down with a clean lint free cloth.