Chapter and part headings are centered horizontally, followed by an additional (double spaced) blank line before the first paragraph. Begin each Part or Chapter on a new page. See William Shunn's manuscript formatting guide for more details.
I agree with Cogito's link in the way that it is set out, also having it double spaced means that it is easier to read when you review it. The only thing I have to say about it is that you should use an exact word count.
It is okay now to give the exact word count according to Word. Everyone uses that these days--calculations were for typewritten manuscripts. And the font doesn't have to be Courier, in the UK anyway.
who is 'everyone'?... as far as i know, for book length works, pros still round off the word count when providing it to agents and editors...