Especially once you've gone over your work several times, it can sometimes be difficult to see what is on the page. You lose focus. You read sentences half-way or you read too fast and you miss mistakes. When you read your work out loud, you're more likely to catch some of these errors you missed. It can also help you catch awkward sentences, but also will give you a sense of how it feels. This means making changes to the big picture. By revising your work, you take your rough draft and turn it into something that is better developed and tailored to its purpose. Don't worry about all the little things in this phase. What you want to do is to make broad-stroke changes.  Rethink structure, such as paragraph order and focal points. Elaborate where needed. Cut out anything unnecessary. This is different than proofreading. When you edit, you go through your work with a sharp eye, crafting style and coherence. When you edit, you also correct awkwardness in your writing.  Check for cohesion and flow by assuring each sentence connects to the sentence which follows. Make sure your information is accurate. Use stronger language. Seek ways to build sentences that are more clear and exact. It can often be very helpful to have someone else read your work. This can give you a new perspective on the ways you present your ideas. Talking about your manifesto with someone else can also help you to develop stronger ideas and more specific articulations of your aims. Proofreading is very important to help maintain your credibility. This means looking for all the little errors you made while you were writing. These can make your work look sloppy and not credible. Do this once you're sure you won't be making any more big changes. It will be the last stage of your editing process.  Look for incorrect words. Find punctuation mistakes. Fix all the little errors.

Summary:
Read it out loud. Revise your work. Edit your manifesto. Ask someone you trust or respect read it. Proofread your manifesto.