First, check out what is a query letter to get the basics.
These are pointers I’ve given in forums before. Without much further ado…
Check out other queries.
Do a search on successful queries and note which ones interested you. Note the pace, word choice, length, etc. Then apply that insight to your mini synopsis. Bear in mind that Your query is NOT those queries, your book is NOT those books, and your voice is NOT their voice. Be you on a page. Write in the voice of YOUR book.
There are a lot of writing websites with query critiques. It’s worth a glance to see what got flagged. Also, search for QueryShark and get VERY familiar with the site.
***When checking out “successful queries,” keep in mind that some agents hate the query but love the pages, or love the query but hate the pages. Just because a query was present in the letter that landed the agent doesn’t necessarily mean it worked. Therefore, to get a broader sense of a “working” query, read at least ten.***
Start with the first inciting incident.
Inciting incident is a fancy way of asking, what event throws the main character (protagonist) into action? Usually it happens within the first fifty pages. Depending on your book, it might start later.
Apply the formula.
First paragraph: the hook. Who is your MC and what happened to throw his/her life out of whack? Make it interesting.
Paragraph 2 (and/or 3): the development. Further explain the conflict/issue. You might introduce the antagonist here or in the hook, or not at all (not all queries have one). Keep in mind, this isn’t a place for backstory. Make it riveting.
Last paragraph: what I call the closing hook. AKA, the stakes. What is MC’s task, and what awful thing will happen to MC if MC does not complete his/her task? Make it powerful, make it intriguing.
Let it sit for a few days.
Then read it. If you pause to reread a sentence, then that sentence needs to be rewritten.
Have someone read it.
No, I don’t mean just anyone. Someone who knows queries intimately. Other writers, agents who’re offering, someone with expertise. Wait a day or two to mull it over, go back to their suggestions and apply which ones you agree with. In that day, you might find that you actually agree when you didn’t at first.
Send it to a few agents.
Then send it to a few more. If you get a certain number of rejections (how many is your choice), then either the pages are the problem, or the query. You might consider fixing your query.
This is just a little advice on where to start. Keep researching until you feel you’ve got a great grasp on the whole picture. And, READ QUERY CRITIQUES!!! Read the original query, read the comments, read the final query. Read everything. When you see how it transformed, what got flagged, and word choice, you’ll save precious time and avoid a self-induced head slap.