It seems like these discussions always go the same. "Leverage your existing reputation and contact list". That is completely missing what the OP asked.
How do you start from nothing?
Indeed, a lot of people here don't really want to help, they want a quick ego stroke.
Anyway, here's what I would do.
If you have nothing to show I would start with small things that have big ROI on "showing off".
Help some non-profit with their website for a few hundred (or for free) make sure to establish a warm relationship and then you can say "I worked for this Church, the Red Cross, University of X" or something like that, "you can contact X for more details", if you did something that's public then show it off! It's all about that.
If you're after technical jobs, contribute to some open source projects, just close down some tickets get on the AUTHORS file and then you can say "I contributed to Firefox" or something.
If you think a bit about it, you can align this experience towards the job you want, a concrete example:
You want to land an incredibly well paid job on the AI/ML space.
* Check out tinygrad, they have a list of open issues with bounties, meaning that you wouldn't be working for free.
* Solve three or four of them. Do it well, documentation, follow procedures, be autonomous, etc...
(by this point I would offer you a job and I'm sure many others would as well, but that's specific to that particular niche)
You get the idea, adapt to whatever you want to do.
Edit: I just realized you're talking about a firm not yourself as a developer, lol.
Most of it still applies I would only add that many people have told me that expos and industry events are where they get like 80% of their clients, it is really worth investing in them, set up a small booth on a local SMB fair or something.
Established nonprofits usually have established providers and usually pay them well over the long term.
They know what they need and pay what it costs. They know what they don’t need and avoid it…
OP didn't mention anything about starting from nothing. In any case, even with the newest company, somebody must have contacts: former co-workers, friends, colleagues, peers, etc.
In any case, sales/marketing/advertising is manifold. Have a website with a good explanation of your services and examples of projects you've worked on. (No past projects? Create some, even if just demos/proofs-of-concept.) Make it easy for visitors to join your mailing list for updates (maybe offer them a free whitepaper for joining). Find out where potential customers hang out (whether online or meatspace) and hang out there with them. Ask questions. Discover their pain points and address them. Look for sites that list contracts (yes, there are a bunch) and comb through them carefully and frequently. Don't try to be all things to all people, i.e. focus on customers and projects that can use your strengths. Create a free product and release it to get customers, then try to upsell them on extra features.
Knowing what type of software you specialize in would make it easier to give more specific suggestions. Government contracts are way different from websites for mom-and-pop shops which are themselves way different from embedded systems. Each of those spaces has their own challenges and quirks.
You start building contacts and reputation. This can also be called “Sales”.
Since that’s hard work, you could consider this answer “hard work.”
But only the right kind of hard work makes a difference. If you can’t be directly rejected, it doesn’t count as sales.
You can always hire technical talent, later. Sales is all that matters and is a people business.
By building a good reputation and contact list doing salaried work. Usually you do a good job there, make a bunch of stakeholders happy, and then you have a chance at spinning off on your own.