For those looking to get an agent, writing a query letter is a big change from creative writing to marketing copywriting. Your query letter needs to sell yourself and your book in a small space.
Your first task is to find an agent to query. If you are UK-based or looking for a UK agent, the Writers and Artists Yearbook is invaluable. You can also use Query Tracker and Manuscript Wishlist (that is active on Twitter MSWL). Once you've found an agent that is suitable for your work do some research on them. Find out what they're interested in and what they work on. Don't use any information that is clearly private or personal though. It is good to keep track of agents queried in a spreadsheet.
Once you know who your agent is and what they like, it is time to start writing. Unless otherwise specified, write in 12-point single-spaced Times New Roman or another sensible font.
Your introduction should contain all of the important details about your manuscript. You should address the agent by name and waste no time telling them what your genre and word count is. If you have written a heart-breaking sci-fi romance, they need to know that from the off-set. The other thing your introduction has to do is make it relevant to this particular agent. There are hundreds of agents interested in sci-fi. Why is this one interested in your sci-fi. Have they recently expressed interest on Twitter for sci-fi with a strong female lead? Does your sci-fi check that box? Tell them. The paragraph should say what it is and why it matters.
The following segment of your letter should be about your book. You want to give the agent your full synopsis. But it has to be brief. Getting the action of your 75,000-word book condensed to two paragraphs is difficult. Get people to help you. Have you built any great connections with beta readers? Do you have a writing circle or a critique group? Write the first draft of your synopsis and invite people who know your manuscript to tweak, comment, and polish it up. This synopsis is different to back cover matter. You're not enticing the agent with just the hook and a cliff-hanger. The agent will want to see a broad overview of the plot, including how it ends. Everyone living happily-ever-after in a new space colony and the protagonist's love interest getting trapped on a distant planet unable to return are two very different products to market.
The final section of your query letter should be about you. Is this your first novel? Is it your third? Have you been published anywhere before? Did you get featured in a literary magazine? What inspired this story? Do you have any other interests? Have you got a platform, existing followers, people invested in you or your writing? Tell the agent here. Get it all down on paper. Then ask a friend to look at it and tell you if any of it actually isn't interesting. Tweak it until it works.
Once you have this written, the next step is to go back and find another agent and do it all again! Not entirely. Your synopsis will stay largely the same. It's a good idea to tweak it to suit the agent's interests. If an agent has mentioned they like cities and urban settings, mention that your book explores city environments in sci-fi settings (if that's true, of course). You will also change the first paragraph to be specific to this agent. You might tweak your last paragraph if an agent has mentioned what they look for in an author. If they've said they like books inspired by true events and your manuscript fits that, tell them.
The final advice is to check each agent's submission guidelines before querying. Some will want a short sample. Some will only want the letter. Some have specific ways they want these things to be formatted. Don't let yourself fail simply because you didn't check the rules!
Querying agents is long and tiresome. It can seem like an endless battle. Many authors have queried hundreds of agents before getting signed. So, don't give up. Keep track of what you've done but don't let the numbers get you down.