1. Start with a piece of cardboard or poster board paper larger than the fireplace opening.
The cardboard needs to have at least one factory/straight edge (as shown on the bottom here).
2. Cut a hole large enough to reach through in the center of the cardboard.
3. Place the cardboard (factory edge down) as tight as possible against the fireplace opening.
4. Reach through the hole and trace/scribe the shape of the hearth opening onto the cardboard.