It's not a secret that it's better to be submitted for roles by your agent or manager than to submit
yourself for a role. They actually have access to castings that you will not see anywhere online. So if you're not currently represented, self-submitting can be a little tricky. Some casting directors don't mind self-submissions, if you're right for the part, because it makes their lives easier, but most won't like it because looking at every single self-submission can become overwhelming. We're talking of hundreds of possible auditions for a single role. Can you imagine sitting there all day and listen to actors who may OR MAY NOT be right for the part read the same script over and over?
So here's something you should always consider: MAKE SURE YOU ARE RIGHT FOR THE PART.
If you have an agent and they submit you for a part you may feel you're not right for, well, that's a different story, they may know something you don't, but if you've emailed a casting director and they called you in and you read the part for a strong and serious character event though you normally play the comic sidekick, this might be a mistake. If you waste their time, they might remember you next time and not even give you a fair shot. Same goes when you're applying for a job via Actors Access, Stage 32 or Backstage or any other online source.
Still, if you're planning to self-submit via email, make sure you know what the specific casting director prefers before submitting yourself. If you're a newcomer and you want to self submit, you may want to get to know casting directors personally first. If they give you their contact information you can submit yourself for a specific role with a concise email. So again, only email casting directors if you genuinely think you're right for a specific role and you know them and know they'll be willing to accept self-submissions.
Always remember to attach your headshot, demo-reel, resume, list your website and PLEASE, do personalize the message. If you don't know what to write to them, look them up on Facebook or Twitter and make something up, but if your email reads like cold mail, it will be treated as such - deleted, or worse "mark as spam".
My advice would be to send everything mentioned above as a link instead of attachments. I personally love the bulletted approach where each item in the list is a hyperlink word of what's being attached. Then if they click on a link that says "Headshot" it will open up your headshot in the browser window. Same for reel and resume and obviously, same for the website.
As I mentioned above, in order to get studio or network-backed auditions, you must be submitted by your SAG-AFTRA certified agent or manager. If you don't have either, you're out of luck because actors don't have direct access to these high profile auditions. Only agents and managers are given access to these auditions via the "breakdowns" and they use your Actors Access profile to submit you. From time to time I see people mentioning selling you access to that list on social media, I actually was tempted to buy it to see how it all looks like, but the person disappeared because... well, because it's illegal to share those, and maybe they've decided it's not worth losing their job over it.
Anyway. If you have an agent, you might be getting the high profile gigs as breakdowns, otherwise, self-submit yourself online. We've listed bunch of websites that allow self-listing on hiredactor.com. Finally, if you know a casting director, and you know they accept self-submissions, send them an email, but make it easy for them to access your information. Don't forget link.actor - this too can be well utilized as all in one base section for your emails.