I'll caveat right up front that I don't have an agent yet. If I did I likely wouldn't be writing this. But since I'm at the end stage, where I'm submitting query letters and getting a few positive responses, it seemed like the perfect time to do this. Also, I'm aiming at fiction novelists. If you write short stories, or non-fiction or anything other than novel length fiction this isn't really for you.
There are a few things you will need to start the process--a manuscript, a query letter, a synopsis, and a list of agents.
First and foremost you need a finished manuscript. You can prep the other stuff, but before you send the first thing to a potential agent you need to have your MS completed, reviewed by alphas and betas, and edited as well as possible. It should be polished to a fine sheen, because this is the thing that will cement the deal. Even if you write a great query and a great synopsis, if your manuscript is subpar the agent is going to pass. Make all the arguments you want about story trumping writing or vice versa, but it will still come down to making that agent fall in love with your novel. Work hard on getting the first part of your story to really grab attention and showcase your voice, because it is the first thing the agent will read. Well, duh, but really, it needs to shine, because they always want the first pages--anywhere from ten to thirty or maybe the first three chapters. I'm just going to assume you did this part and move on. Don't make me regret it.
We've toiled on our manuscript for months or years and it's ready to be seen, so the next step is to work on your query letter. The query letter usually includes an introduction, a pitch, and a short bio. It should be double-spaced 12 font, and don't get fancy with the font. My preference is Times New Roman for this stuff. Ideally, it will be all on one page.
The concept for the query is to pique the interest of the prospective agent. Because of this, you need to personalize each one for every agent you query. So, while working on your pitch, you need to start developing a list of agents. Agent Query is a great resource to start (www.agentquery.com/) or you can just Google "your genre" and "agent" to see what comes up. Chuck Sambuchino has a great list at Writer's Digest (www.writersdigest.com/editor-blogs/guide-to-literary-agents) and he updates it with new agents as they emerge. Preditors and Editors (pred-ed.com/) provides a wide-ranging database for lots of stuff, including which agents to avoid, whether or not they charge to review your work, legal services, and convention lists, just to name a few things. Agents that charge reading fees are warned against, as reputable agents do not charge a reading fee. Absolute Write Water Cooler is another good resource for reviews of agents. (www.absolutewrite.com/forums/)
Take the time to do some research on the agent you select. If you have a connection mention it. If you met them at a con remind them. He or she will usually want your word count and genre listed in this introduction section. If they don't care about such things the agency listing should say. Or if you find an interview online they might mention specific likes or dislikes in a query. Some have done video blogs or live chats where these kinds of things are discussed.
Their agency website will have information about the submission process and their agents, especially their genre preferences, and usually a little bit of personal stuff, like favorite writers. Also take a look at their client list to see if you're a fan of one of their authors,or even if they have any yet. If the client list isn't mentioned on the agency page you can find out on Query Tracker. (querytracker.net/) It's also a great resource for finding agents. The agency website will also have the submission guidelines. Follow them. Every agency is different. Some will have a simple submission worksheet that will be very limited, asking for contact info, a short bio, and a pitch. Others will take 30 pages and a query letter. More will want a full synopsis and first ten pages of your manuscript with the query letter. From my experience, most will want a query, a short synopsis (1-2 pages) and the first ten to fifteen pages of your MS. Pay attention to the details; they're usually quite specific. They will often have one email address to sent your query to, and might share within the agency if the particular agent you targeted thinks you might be a better fit with someone else. Some will have you contact the individual agent directly.
These days most prefer an email with everything including in the body of the email. There will usually be a clear note not to use attachments. Your email will get deleted without being read if you don't pay attention to this. There are still a few holdouts that will only take snail-mail packages.
The meat of the query letter is the pitch, and it needs to have a hook. The sole purpose is to get the agent to ask for your manuscript. This is the part you are going to spend the most time on. You want a one to two paragraph lure for your story, including the stakes and the thing that sets your story apart. I recommend sticking to the protagonist, and maybe the antagonist, and provide the agent with something enticing. There are lots of good places on the web to get help with these. I started with Query Shark (queryshark.blogspot.com/,) which a lot of my friends used as well. Jane Friedman also has a good page for these. (janefriedman.com/2014/04/11/query-letters/) Writer's Digest has a nice Dos and Don'ts page. (www.writersdigest.com/online-editor/the-10-dos-and-donts-of-writing-a-query-letter)
Once you've created that pitch, you can test it out at several blogs. These two will post on their blog and publically critique: Writer Writer Pants on Fire (writerwriterpantsonfire.blogspot.com/p/query-critiques.html) and Kyra Nelson. (kyramnelson.com/query-critiques/) At Agent Query, it's more like a club. (agentqueryconnect.com/index.php?/forum/2-aq-connect-query-critiques/) There are sites popping up and folding pretty regularly, and at the time I posted this all of these were still active. There are contests all the time, so don't be afraid to do some searches on the web when you've drafted your pitch.
For your bio, keep it short, but include any other work you've had published or any experience you have that is germane to your story, like you're an attorney and you write court dramas, that sort of thing. School history is a good thing to include, especially if it pertains to writing, for example an MFA. These things aren't required and if you are a fledgling writer with no writing credits just keep it simple. That won't stop them from reading your work. A great pitch will overcome a lot.
The next thing on the list is the synopsis. The synopsis is a full summary of your story with all the spoilers and secrets revealed, including the ending. There are resources online for how to do this, but essentially it is a scene summary of the conflict in your story. Carly Watters has a nice "how to" on her blog (carlywatters.com/2013/11/04/how-to-write-a-book-synopsis/). Jane Friedman also has a great helper (janefriedman.com/2011/10/25/novel-synopsis/.) I would make several of these of varying lengths--a one page, a two page and a full, which could be up to ten pages. These are usually single-spaced, and nobody has told me different. What they are looking for here is can you tell a complete story. Do your best to keep some of your narrative voice in these.
Lastly, I created a spreadsheet in Excel to track who I sent to and when, what they asked for, and a place for responses and comments. I used a default rejection if I didn't hear anything back after two months. You could always resubmit if this happens. Most advice I've seen is wait at least 30 days for any follow-up. Some agencies welcome trying other agents in their house after a month, and some will tell you not to bother.
You want to keep your manuscript on sub until you have success. My goal was to always have it out to at least four all the time. If someone asks you for an exclusive there needs to be a reasonable timeframe included, 30 to 60 days is normal. Don't let someone lock it up perpetually.
I actually have my MS with an agent right now, waiting to see if he will want to represent me. If he says yes, then the next step of the process starts, seeing how well you work together. Just because you get an offer of representation doesn't mean you have to take it either. Read the fine print on the contract. Obviously, the more reputable the agency the less you have to work about this, but you still need to read the contract carefully.
Keep your chin up. There will be lots of rejection. Some of it will be nice, some will be generic, and some will seem to simply ignore you. Stay professional, and in the meantime, work on the next project. I wish you all the luck in the world.