Some writers actually find it easier to work out of order, choosing to start writing about the thing that they understand the best, feel most comfortable with, or find most interesting. While some steps should be completed before others, sometimes you may start writing about one part, and then switch to another when you run out of ideas. Then you can come back at a later time with new ideas or insights.
At some point you will be able to spread out in front of you all of the sections that you have written. You will be able to sequence them in the best order and then see what is missing and should be added to the proposal.