Submission guidelines vary. Some want a cover or query letter, a synopsis, and either the first three chapters, 10,000 words, or in some cases, the entire manuscript. The manuscript needs to be as polished as possible before submission as you never know how much time you might have, although I understand if you're sending to agents, you'll have between weeks and months to continue to prep.
For my publisher, I had to submit a query letter and the first five pages of the manuscript. They said ninety percent of the submissions they receive don't make it past this point. After I sent in my submission, I kept polishing the rest of the manuscript. Good thing, too--five weeks later they requested the entire manuscript. Within a week of receipt, they offered me a contract and told me to stop working on the manuscript because the offer was based on what they had already read. Any changes would be worked with their editor. That's why I immediately took it off TNBW as I wouldn't be able to use any further suggestions. Hope this helps, but if you have further questions, please let me know.
