As has already been said, by far the best way is to work for a company whose product is open source. I would find one that is fully committed to open source though, not just partially. By that I mean, a mainstream, money-making product of theirs is licensed appropriately (AGPL, MIT, etc). Going with a company that open sources a few libraries here and there is not a good strategy.
That said, I have friends that have been able to spin open source roots into money, in one case really good money. Their paths were:
1. Find need not being addressed
2. Write open source solution for said need
3. Promote open source solution to drive adoption (yes, marketing is important in open source!)
4. Create training courses/material on solution
5. Sell training primarily to corporations and sometimes individuals for $
The other path has the same first 3 steps, but instead of training:
4. Establish a "support" department
5. Sell support to corporations
There's also the "open core" approach where you put enterprise features behind a pay-gate but give the basic product out as standard OSS. The biggest challenge here is that your business strategy will ultimately be in conflict with your community member's interests, which inevitably results in sacrificing one for the other, usually sacrificing community needs for the business. For example, consider you have SSO gated behind payment, and someone sends you a PR that adds SSO to your open core version. I would personally avoid this model unless you enjoy inner philosophical conflict with guaranteed losers and having to exert tight control on other people.
The support path requires your product to be a lot more mature than most people usually think as most corps will expect things like SSO, audit logs, and other enterprisey features. You can usually find customers who won't insist on that, but any of the medium to large fish will.
The donation model can be sustainable and even pretty good if you find the right niche, though expect to put in a lot of time/effort early on before you get many donations.
Lastly you could try the free-as-in-speech but not free-as-in-beer approach where your software is GPL but not free. Davis Remmel has used this for rcu[1] for example (which if you have a Remarkable Tablet you should absolutely purchase rcu!). Personally I love this model and am highly inclined to purchase when prices are reasonable, but it's not for everyone.
Regardless, thank you for being interested in contributing to OSS! I think most people are largely unaware of the importance and significance of open source in our current world. It is one of the greatest gifts you can give, and enables people to do things they otherwise wouldn't be able to do. Essentially everything we have nowadays is powered at least in part by open source.
[1]: https://www.davisr.me/projects/rcu/
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