I get asked something like this at every convention and often through email or DMs: "Could you give advice on pitching ideas to companies?"
Presumably, because I've been in a position to be on the receiving end of the pitch process. Well buckle up, we're going on a short ride!
This advice is free and it's relatively simply to follow. Your mileage may vary (so many car metaphors), so I hold no responsibility here.
#1 - Check their website to make sure they accept submissions. Some will open submissions from time to time, so you may have to wait a bit.
#2 - See how they want to receive submissions (ie: link to a pitch package, attached to an email, mailed copy of package) and in what format they want that package submitted (PDF, a Word document and jpegs, physical print...that sort of thing).
#3 If they they are not open for submissions right now, don't send them a submission anyway. Open a conversation with an editor if you can. Ask them if you can send a package of your work their way and follow up with them in a couple of weeks. If they say yes, do just that.
#3 addendum
Don't inundate that editor with e-mails, messages or phone calls. Try to remember these people tend to be very busy. They may not respond because they have to prioritize their workload and communications.
#4 Make a pitch package! Include a pdf of several FINISHED pages as well as bios of the people involved (qualifications/experience). Keep it short, but include a synopsis of your story and whether you see it as a limited series or graphic novel.
#4 addendum
When sending pitch packages, always acknowledge that you realize they are busy and try to remember to thank them for their time looking over your pitch. It certainly can't hurt to let them know you understand how busy some of them can be.
#5 Follow up with them, but not incessantly. If you have no reply after a couple of follow up inquiries, take a hint. They just may not have time to write you a PFO email. Give that bucket another kick around 9 to 12 months later. 6 to 9 months minimum.
#6 Keep positive. You may not get a response or feedback, positive or otherwise. When you do, it will more than likely be "no thanks". Just keep working. You'll never get any better if you don't keep at it and challenge yourself.
#7 Many will crowdfund their own work at this point. It's a great way to gauge interest and get feedback from your target audience. Also, if it's successful, you'll have a physical copy to promote at conventions and present to editors & publishers at shows or signing events.
These aren't necessarily ways that everyone in the industry would agree upon, but that's kind of the point. There really is no one set way to get your pitches seen and published. There is no single way to find work in the comic book industry. Just do your best work.
Good luck!