Given that you can successfully create big account on IG (100k, 200k, 500k), now how do you start working with well-known companies to create sponsored ads? I am not talking about doing shout-outs for small mom-pop stores, I am talking about fortune500 companies and well-known brands?
Do anyone here have experience on IG working with legit companies?
you can contact them directly by sending an offer
I think that @Luca has the point. You should contact them directly with some good idea and prepared offer, or at least some options for them. But try contacting them on social media, big companies are very responsive there and they have people in charge for managing social media. You will get them faster there, than on some info emails 
Affirming the above. Said companies are very responsive on social. Especially when your account is large.
If you had 5k followers, they’d be excited already for a deal. Imagine the sizes you are talking about.
You’ll have the leverage if you approach them with a sound offer like mentioned by @Bianca
If not, leverage their competitors in your offer
Well wishes to you on that!
Those are some impressive numbers @willowtree, you should be able to monetize your accounts fine. I agree with previous suggestions, try contacting them directly. I hope you’ll be able to get a good deal.
Does this work for big companies? I feel like this works for small mom-pop stores.
I don’t know if it does for big companies, unless you’re an agency or popular mega-influencer. Then again, a lot of them have affiliate-type deals that you can get involved in, but it’s not the same thing really.
Come to think of it, in the past when I had a 20k account no matter who I reached out to, mind you this was for collaboration and providing them free content, they all said no because they wanted more. So even “free” has a limit, which is weird when you consider the cost for them to get involved would be nearly nothing beyond shipping a few cheap samples to use.
Anyhow, it’s worth a shot cold emailing or DMing to see if you get a response, right? Especially if you’ve got big pages, not a tiny 15-20k like I tried with.
also big ones, you will find them much more responsive then the small ones
I’ve dealt with a few large companies for the events I run, so hopefully I can provide some insights (£2m, £30m, & £750m revenue). First a disclaimer: it was the events they were interested in, not my social accounts. That said, hopefully I can spread some light on how they liked to work and how the approach went.
£2m - a chance meeting with the founders, an introduction, and them being interested in and looking at my sponsorship pack. These folks were more interested in putting physical products in people’s hands, not so much social. I think squeezing much money out of them would be difficult unless I had more followers than them on my IG at the time (they had 25K) or a different type of event.
£30m - Surprisingly simple marketing dept, not a huge social presence. Sent an email to the CEO who I had met before (I made it seem like we were better friends than we were and just hoped he didn’t remember that we’d only spoken for a very short time!) They wanted social proof for their online presence as we’re quite trusted in our niche and had events where they don’t have stores.
£750m - Big players and funnily, the most willing to splash money quickly. Intro via a mutually known party to their marketing dept. Again, sent through event overview and sponsorship pack. They initially wanted to drop £5-10k at the drop of a hat - we’re talking a 2 week lead time here - that even spooked us! They didn’t care about social, but it would have been a public relations and corporate responsibility stunt for local favour.
So, what can I say?
I know this isn’t directly useful, but hopefully there’s something there for you.
If you have large accounts, my recommendation is to focus on how you can provide social proof, and start optimising a B2B strategy aimed at these companies.