After you have generated ideas, made connections, and organized your initial thoughts, you are ready to begin writing. In the first draft stage, write as quickly as you can without concern for grammar, spelling, or word choice. You can review those concerns later in the process. While writing your first draft, you may go off topic -- make note of it. You may also get stuck at some point -- relax, and keep going. The point is to write, write, and write!
Check out the pdf below to see a section of Anne Lamott's Bird by Bird: Some Instructions on Writing and Life. She explains the usefulness of writing a first draft, and allowing yourself to compose a not-so-perfect version of what will be your final product.
