Start by writing down as many terms, phrases or words that relate to your topic, no matter how silly they seem. If you can, write sentences but don't worry if they make no sense. The point is to keep writing.
Take a break after at least 15 minutes of freewriting. Remember, it may take a while to get your thoughts moving in the direction you need them to go. Return to the topic after five minutes and read what you have. Try to connect some of the words, phrases or terms to each other to help develop your topic. Look for anything that pertains to your topic and focus on it. Ask yourself how the other freewriting terms, phrases or words relate to those you chose.
Write down those pieces of freewriting that are most pertinent to your topic on a fresh sheet of paper. As you transfer the information, you may find that it engenders questions about your topic that could start the process all over again. Let those ideas carry your theme along for the next 10 to 15 minutes, then stop and read this section. At this point the material should begin following the topic and maintaining a focus.
Repeat the freewriting process, gradually narrowing down the scope of your theme, until you have enough thoughts to summarize your topic. In some cases, you may wind up with two or three different trains of thought. Summarize each one. This makes it easier to proceed to the next step of the writing process: writing your first draft.