I have some experience with this. There are multiple ways to get a services firm on its feet. The ways I've seen engineers be successful doing this on their own, without capital investment, generally involve:
1. Growing a public profile for the firm (low effort: blogging/content; medium effort: strategic open source contribution; high effort: direct outreach community building, setting up events, doing trainings)
2. Landing a couple early customers and bending over backwards for them
3. Using endorsements, case studies, and word of mouth from those customers to grow the business.
One strategy that can work: find ways to specialize (ideally: not on specific technologies, though AI might be an exception at the moment; specializing for specific verticals) to make landing and expanding within specific segments. We did this, for instance, with intitutional financial technology (trading, FIX, etc). The idea (early on) isn't to stay with that practice focus area, but rather to streamline that 1-2-3 process.
What I've seen tried, but haven't seen work: cold-calling, direct email, account management style sales.
The good news here is I'm not sure there's a whole lot of sales domain knowledge you need to execute this strategy. The bad news is that there's no substitute for hard work in it.
Probably the most important things to know going into this:
* Your rates need to account for "feast or famine" cycles in your business, meaning they should be drastically higher than your current FTE comp backed out to a bill rate; 2x is a floor not a ceiling. The first year of a services firm is usually terrifying (at least, on the pipeline calls) and if you've set things up right the reassurance you have, when there are no projects set to close in the next 8 weeks, is that you only need to be 40% utilized to match your previous FTE comp.
* If you've positioned the firm correctly, the big problem in your market should be that buyers don't have enough options for firms to do the work; for instance, lead times for new projects being 3 months out would be a good sign. This is market research you can just go do! I transitioned a few years ago from a seller of services to a buyer and it was eye opening; we hadn't succeeded, in my previous firms, by hustling, but rather by positioning ourselves in places where customers were already tearing their hair out trying to find a trustworthy firm that could start a project within the next quarter.
Hope that helps!