Step #1: Butt in chair.
Step #2: Chocolate. Or cookies. Or both.
Step #3: Write the names of your main character and antagonist. If you can't figure out who's who, slap yourself across the face. Twice.
Step #4: Write a one-sentence hook that will entice agents/editors to read the rest of your query. For example: "Seventeen-year-old Tom Cruise doesn't believe in aliens--until a mysterious cult jumps him on the red carpet while chanting 'Scientology rocks, biatch!'."
Step #5: Write two-three sentences describing the main character. Physical appearances need not apply, unless it's important to the plot. In that case, slap yourself again. You're ripping off J. K. Rowling.
Step #6: Think about what main character wants vs. what antagonist wants. Write as much as your mind can come up with, then delete all the bull#!^% until the essentials are left.
Step #7: Explain what's at stake if main character loses. This makes agents/editors care about it. They want to hug it, take long walks on the beach with it, maybe even cook it dinner. And that, my dear query wuss, is the goal.
Step #8: Make sure your "voice" is reflected in your writing. If you're thinking about printing the query and talking to it, don't slap yourself. I'll do it for you.
Step #9: Convince yourself that your query rocks. Wait fifteen minutes, then convince yourself it sucks. Wait fifteen more minutes, then go back to thinking it's awesome. Repeat for at least three months.
Step #10: Hit send.
Sounds easy, right?