Assemble your writing materials. This could mean getting some paper and a pen or turning on your computer and opening your word processor. Many people find that they have more success with this exercise when they write by hand, on paper. Do whatever you are most comfortable with.
Remove yourself from distraction. You need 15 minutes to do this exercise. You should be in a quiet place, free of distractions and noise.
Chose a focus or topic for your freewrite. If you are stuck on your novel or short story, choose to write about that. If you are trying to start a new project, you can choose to write on a topic, or just decide to see where the writing takes you. If you are having a particular problem in your life that you want to work through, choose that as your topic. You do not have to have a specific topic going into a freewrite. You can simply choose to write whatever comes to mind.
Write! Do not lift your pen or stop typing for the entire 15 minutes. This is largely the point of the exercise. It helps get past the jumble of thoughts in your brain or the voices telling you that your ideas are terrible. If at some point, you get stalled and run out of things to write, this is what you will "talk" about on paper. Write things like "I can't think of anything to write" until your thoughts start flowing again.
Keep writing until the 15 minutes are up. Remember that you are not producing art, or even publishable-quality work. What you are doing is more like taking dictation from your brain. Often, you do write something beautiful or say something important that you couldn't have gotten to without the preceding pages of nothingness. This is the value of the freewrite. Repeat this exercise whenever you get stuck, or daily as a writing warm-up.