Follow the guidelines as closely as possible and try your best to make your book as appealing as possible. Remember that the people reading your submission have never read your book and probably have never even heard of it, so these are first-time non-readers. Don't be afraid of spoilers, either; the submissions people are OK with that.
As for the cover letter part, look up good cover letter stuff online, but don't sweat that too much. The submission details themselves are the part that will make you shine, so as long as the cover letter is nice you're probably fine.