So you've found an agent you like and have researched their reputation and tastes. Now comes the easy part. You just write a stellar query letter and win them over will your brilliant novel. No problem.
Ok, maybe a slight problem.
Now, the first thing you'll need to do is to learn how to format a query letter. I'm actually going to skip that section, because you can find a plethora of information on the web about it. Believe it or not, I've found the article on SoYouWanna.com to be the most clear cut, even if it is written towards nonfiction writers. However, don't get too stressed on "proper" format. Everyone has a different opinion on the format. The most important thing is that you include all the proper information. However, if you see that an agent has written about the proper way to write a query letter, be sure you format your query to them in that manner.
Since you can get information about proper query formatting from other sources, I will discuss how to write the query letter. Not the mechanics, but what actually to include. That's the real hard part, isn't it? Well, to do it right, it all goes back to a proper attitude.
Remember what I've said about the agent/client relationship. You are a headhunter. That's your task when you send out a query. You're saying, "Hi. I know you graduated from Yale and are highly qualified to work anywhere in the country. This is why you want to move to Scranton, PA and work for me."
Since few of us are headhunters in our daily life or have written job opening descriptions to post on monster.com, then we'll need another comparison. Although it completely ruins the "you're the person doing the hiring" message I've been expressing, the best comparison that you may be familiar with is the resume cover letter.
If you've ever applied for a full-time job, then you probably have written a resume cover letter. If you haven't that's ok too. The idea is simple. Essentially a query letter, like a resume cover letter, is self-marketing. It is a single-page description of "you" as it applies to the person to whom you are sending it.
When writing a query letter, write with confidence. You are exactly the person they need. However, if you are the right person, then you don't have to tell them that. Instead, show them. Yes, it's that same show vs. tell issue we all deal with when writing. It doesn't change when it comes to query writing. Show them why you are what they want. You know they're looking for strong female leads, and you have one. Don't say, "Jane Doe is a strong female lead." Instead, show that she's strong. Tell them the diversity she faces and how she overcomes it.
(Ok, not to interrupt the flow here or anything, but I just typed "how she overcombs it." If you have a story about a girl with a combover, I want to read it."
Another important part about writing a good query letter is to inject it with voice. Not that writers with voice necessarily have any idea how they have voice. What you read online about it is really unhelpful. What's voice? It's the tone of your story. Is it humorous? Arrogant? Dark and sinister? Imagine a voice actor reading your manuscript as an audiobook. How would they read it? Find that element in your story and channel the attitude you had while writing the novel. Another good tip is to find particularly "vocal" portions of your description/narrative and use them as part of your query.
Here's a good case in point. Let's imagine you're JK Rowling, writing a query for Harry Potter and the Sorcerer's/Philosopher's Stone to Johnny McAgent:
Dear Mr. McAgent,
My Name is J.K. Rowling, and I have written a manuscript titled Harry Potter and the Philosopher's Stone, the first in a 7-part YA fantasy series about a boy who goes to a school of magic. The manuscript is complete at 75,000 words. Thank you for your time.
Mr. and Mrs. Dursley were perfectly normal, thank you very much. They shuddered the think what the neighbors would say if they knew that their good-for-nothing nephew, Harry Potter, was, well... as unDursleyish as it were possible to be. They'd never told Harry about his abilities or the real reason for the death of his parents, which sent Harry to live with them when he was only a baby. He lived forgotten and unwanted, living in a small room under the stairs and never knowing that there could be more... until the letters came. Finally, a giant named Hagrid, a man simply too big to be allowed, arrives on Harry's birthday to tell him that he is, in fact, a wizard, and not only that, but one of the most famous wizards alive. His parents were killed by the Dark Lord Voldemort, and he was The Boy who Lived. Harry was to attend Hogwarts, a school of magic housed in an ancient and mysterious castle. However, as Harry studies and makes friends, there is a darkness growing. There is more waiting for him at the school beyond flying broomsticks and potion-making lessons. He just hopes he can survive the encounter.
Of course, there would be more beyond this, and I'm sure Jo could write it much better than I did, but you get the point. Now, there are several parts I took directly out of the book:
Mr. and Mrs. Dursley were perfectly normal, thank you very much.
This is a slightly condensed version of the first line in the book.
They shuddered the think what the neighbors would say if... their good-for-nothing...
These two sections are from a passage, but are actually talking about Harry's parents. Still, it works when discussing Harry.
as unDursleyish as it were possible to be.
Again from the same passage, about Harry's parents.
simply too big to be allowed
I don't know that she uses this line in the first book, actually, but I would say use it if she did. A great simple (voice-filled) way of describing him.
The Boy who Lived
That's just too great a descriptor of Harry not to use it. If you have anything clever like this and you can work it in, by all means do. You're showing off, after all.
The rest I just added (though I did steal the last bit from the back cover). However, if you notice, I didn't retell the entire story. I just gave enough for you to become interested and have an idea of what is contained in the book. You know what sort of magic they learn, you see a glimpse of the fun in the novel. However, you also see that there is darkness lurking, which is important too. It's not about telling everything that happens in the book. It's about showing all the book's positive qualities. In this case, the clever language, the suspense and humor, the mystery, and the downtrodden boy with hero potential. We get all of that, but none of the later plot is ruined for us.
Of course, once you have that, you customize for the agent. Do they love clever language? Perhaps you bump up the quotes. Do they like letters to be really professional? Be sure you have a good professional intro like the one I illustrated. Do they like romance? If you have it in the story, then be sure to mention it, and infuse it with whatever romantic voice you use in those sections. I could go on forever here, but hopefully you at least have an idea of what I mean.