How to go independent in your area of knowledge and expertise?

Any tips for going independent and what mistakes to avoid would be much appreciated.

For context, I have a friend who has worked in the smart buildings industry for over 10 years. She is considering going independent but she's on the fence about quitting the security of employment without confirmed clients. Also, it is not unheard of that once you're operating on your own, it's as if your expertise and knowledge aren't as valuable as when you did the same work as part of a well-known company - which I think is a legitimate concern of hers.

If you're still employed and start out independent work on the side, this may breach your employment contract. If you leave employment and you don't have clients, you don't want to make decisions out of desperation.

So, how does one actually get started as an independent consultant?

Should she start sharing her knowledge on Linkedin while still being an employee, to build a "personal brand" and get known for her expertise in the wider circles - and wait for the first "inbound" client before deciding? But this may take years, right? Should she be discreetly reaching out to old clients from previous employments? Anything else - to do or to avoid?