1: Completely clear your head. Take a deep breathe and relax.
2: Don't concern yourself with the rules of grammar. (Very important)
3: Write your heart out for 10 minutes.
Set a fixed time for writing.
The recommended times are 5, 10, or 15 minutes.
Once the time is completed stop writing.
After you are done go back and read the writing slowly.
As you read, highlight some of the key points.
These points may be used for later topics.
Once you start the time it's time to write!
Even if you have to write, " I have no idea what I am typing", get the thought out.
As time goes on you will become better at shaping the X minutes into a single idea.
Remember to ignore spelling and grammar mistakes.
Set a schedule for yourself, daily or weekly are two good examples.
The point here is to free your mind, not to write the next epic odyssey.