It's almost always a 'cover' letter rather than a 'query' letter; few ask you to submit a proposal without any accompanying prose. But anyway, there's hardly any difference between the two. Instead of 'Please find attached a synopsis and the first three chapters', for a query you'd put 'I'd be delighted to send you the synopsis and first three chapters at your request'. Everything else is the same.
- Your premise in tagline form. As short as you can make it. A couple of sentences at most.
- The TITLE, word count, genre and target audience (yes, your title should be in CAPS).
- A short blurb, only a few sentences long, expanding your premise and introducing your main character(s).
- A short biography. Include any writing-related experience. Mention if you have a good social platform following (blog, twitter etc.)
- A few publishing credits. Seriously. If you don't have any, get some. Write a few short stories and submit them. Getting published on a non-paying ezine blog still counts. It shows the editor you're serious and your writing is of a certain standard. And it's the first thing that'll catch their eye when they open your email/envelope - a list in the middle of the page that can only be publishing credits. Sets you above 90% of the rest, gives you an advantage before they've even started reading. Get some.
Please try and find someone to address your cover letter to personally. Many agencies have agent bios where you can find the most suitable agent and address your submission directly to them.
After that, you introduce your novel. There are different ways of working this, but I'd recommend opening with the premise tagline, then the TITLE/word count/genre/audience, and then the blurb. Follow up with "I have attached the synopsis and first three chapters for your consideration" (or whatever they ask for) and end with your bio and publishing credits. I'll say again: those credits will really help you.
Some writers seem to hate writing their synopsis. This is the best way I've found. After your novel is finished, you read it through, summing up each chapter in a couple of sentences as you go. You'll end up with a detailed plot outline (that probably looks quite different to the one you planned in the very beginning, if that's how you work!) Cut out everything that isn't to do with the main plot. Your synopsis should only be two pages, maximum (double-spaced!). That's only 650 words or so. So yes, you'll most likely have to play around with your sentences, cutting and cropping to avoid having precious lines taken up with only a word or two. In your novel, white space doesn't matter - is good, in fact - but in your synopsis you don't have enough to waste.
Also, try to cut out as many characters as possible, leaving only the main ones. You want to mention as few names as possible, or else the editor will have trouble keeping track of who's who. For instance, in my synopsis, instead of calling Lorna's father by his name, ALAN, I always referred to him as 'Lorna's father'. And yes, the first time you mention someone by name you put it in CAPS.
The most important thing is to include all twists and turns, including the ending. The synopsis I wrote for my first novel, ten years back when I was 16, didn't say how it ended, because it was a twist, and I didn't want to 'ruin' it for the editor. So it wasn't a synopsis. It was a blurb. And of course, no editor ever asked to read it all anyway.
Of course, your chapters are the most important thing. Yet if you haven't written a good cover letter and synopsis the editor won't even read this far. So if they do get here, for God's sake make it good. As good as it can be. Don't submit anything until you're sure you can't improve it. And the opening needs to be your very best. You need a main character in a tricky situation, from the very get go. We need to care, we need to be intrigued, we need to want to read on. We need a hook.
Don't open with scene-setting, or narration, or exposition, or describing the lovely fantastical city, or the weather... Throw us in the deep end, right in the middle of the story rather than the beginning.
Then perhaps we'll read on. And maybe an editor will too.