Change One Thing
Start with a sentence. Any sentence at all! If your mind is truly empty, begin with "Here is a sentence."
Whatever your sentence is, write it down.
For the next line, change one word from the line before and write the new sentence.
Then change one word in that sentence to make a new one.
Keep going until you fill the page. What kind of tale have you told?
Here is a longer version of these directions.