I used a chalk marker to write the words because I like the precision, and then rubbed the whole thing with regular chalk after the marker had dried for that smudgy, chalky look.
The quote I used for fall was:
How beautifully leaves grow old. How full of light and color are their last days.
To erase and switch the chalkboard to Halloween, I wiped the board down with a wet paper towel and when it had dried used the same chalk marker and chalk process to replace the quote with an excerpt from The Raven.