Anything flat that you can cut and score works great. This project was done in 3mm (⅛") basswood, but ¼" might be a better choice for a coaster. 

You can also try acrylic or other materials, but pay attention to the material's ability to absorb moisture.

Adjust the settings appropriate to your machine and material for scoring and cutting. Blue lines are score, red lines are cut.

If the six available sayings aren't to your taste, there are 54 more over in the matching “Conversation hearts” project that you can scale up to coaster size. If you use those, be sure to switch engraving to scoring to retain a smooth surface on the coaster.

When ready, just cut your heart(s) out. 😱

Even if you choose not to paint your coasters, you should at least clear coat or otherwise protect the wood, since unfinished wood will absorb moisture like nobody's business.

Painting tips:

1. Spray (or brush) primer onto the coasters. (Unfinished wood will otherwise suck up your paint and require many many coats to finish. Optionally, you also can sand the primed coasters before painting for a nice smooth finish.)
2. Spray (or brush) on paint in different pastel colors (this project used 6 colors). It may take a few coats to get an even color. 
3. When dry, color in the lettering with a red paint pen (or brush).
   - Ohuhu (available on Amazon) makes some great little acrylic paint pens with thin and thick tips on each pen. The thin tip works really well for coloring inside the score lines.
4. Spray a matte or satin clear coat over the coasters to lock in the colors and give them a consistent appearance (helpful if your colors are all in different finishes--they will all take on the finish of the clear coat).

To help insulate the coaster from your table surface and help keep it from skidding, cut a sheet of felt or cork to fit and glue it on the bottom of the coaster.

Place on your coffee table to both a) prevent rings and b) add a Valentine's vibe to your living room. Happy Valentine's Day! ❤️