You'll need to design your card using vector graphics software like Adobe Illustrator or CorelDraw. You can use the attached file as a starting point.
Begin by selecting a font for the letters at the top of the card and typing them at your desired size. Create a rectangle of the same width and your desired height underneath the letters and move it up until the top of the rectangle overlaps with the baseline of each letter. The overlap line will serve as the connecting between the cards and the letters so make sure all of the letters will be firmly attached. Convert the text to a shape and union the shape with the rectangle to create a single set of outlines. Finally, type your message inside the card area itself. Note, the attached design has a border around the outside of the card but I chose not to use a border in the final product.
When you are happy with your design, run a series of power tests on the MDF to determine the correct settings. The attached power test file might help. I used speed/power 60/30 for raster and 10/90 for vector cutting with 2 vector passes. Finally, use these settings to etch and cut your Christmas card!