A query letter is a very very very (I cannot stress this enough) short letter, designed to convey everything that is important about your book (or proposed book) and about you as an author. "How," you may ask "can you expect me to convey the most important kernel of truth that my book is designed around and reveal the most pertinent details of my internal life as an artist and creator in only a few paragraphs?" To which I would reply, "carefully, briefly, and a bit punchy, I don't want to be bored to tears!" I didn't have time to write a short letter, so I wrote a long one instead". Take your time drafting this letter, it's the most important thing you will ever write. Your query is my first introduction to your work. I try to make an effort read sample materials, at the very least a synopsis, but most agents never make it past the query, especially if you're query makes them think "TL;DR." There are tons of guides on how to write a proper query, I would read them all if you have the time. In the end though, there's really no right way to write a query letter. Each query letter is flawed in it's own special way. However, if you try really hard you'll be a step ahead of 90% of the other submissions an agent or editor receives on a daily basis.
Really this should be the first thing on the list. You're going to need it, regardless of whether you have the first three things ready or not. Unless you're unbelievably lucky, I would expect to be ignored for the few months your book is on submission. No one will contact you, repeated follow ups will get you vague answers, or no answers at all. This is not uncommon. Just be patient. Make several submission and wait. If you don't hear back from anyone after six months, then start over and try again. Agents are in the business of representing authors who they believe will create income enough to sustain them both. They are not therapists, they won't ever provide you with closure. If they are kind they will reject you, if they are real mensches they'll tell you explicitly why they rejected you and may even provide you with helpful suggestions on how to fix your book.