Worst mistake - not doing the homework to find out what should go into a proposal. Publishers want to know how you will market,what the competition is etc, as well as the actual chapter and outline stuff.
So take your time, and use Larson or some other author as your guide to make it a complete proposal.

Typos, words spelled incorrectly, anything else to mark you as incompetent.

A couple of books on the subject are

'Book Proposals Anybody Can Write' by Elizabeth Lyon

'The Art of the Book Proposal' by Eric Maisel

Hope this helps!