So this all pretty well comes down to how developed your ideas are:
1. Ready to go and tested
2. A concept with some research
3. Just an idea
So if your number 1, you can go down the route of a few options but before that, if you're in situation 2 or 3 it's very challenging to sell to a big company just a raw idea.
Now getting focus on situation number 1. You have a few ways you can do this:
1. If the firm is a CVC (corporate venture capital) division or firm this might fall under their mandate meaning they would be way more open to taking your idea on and funding it. The secret to it is are you solving a problem that is of interest to them or their industry.
2. Licensing, is an option. This ranges from basically protecting your idea with an NDA and some other legal clauses for licensing all the way to you obtaining IP to assign rights to them. NOT IP is a great idea but I'm assuming you want to just sell them an idea
3. Collaboration with someone who has a great interest in your ideas within the organization. Now you would need to be very discrete in the start with a clear mandate you want to sell. I do want to point out this might end-up them wanting to hire you which might not be a bad idea. Consider it.
So those are three ways to go about it. If you want more ways or details on how to make that work do please reach out to me and we can discuss in more detail.