Write the specific topic your idea covers in a single word or phrase.
Set a timer for at least 30 minutes.
Start the timer and write as much as you can on the topic without stopping until the timer goes off. If you can't think of anything, write "can't think" repeatedly until a new thought comes to mind.
Read around what you've just written. Your ear will catch prevalent patterns and ideas, which allow for easier organization of your thoughts.
Revise grammar mistakes and cut out the "can't think" sentences. Delete anything else off topic.
Edit your ideas for clarity; group similar statements together and connect incomplete thoughts to ideas that complement each other. As you edit, new ideas will come to mind and fall into place in a more organized manner now that you have focus.