Worked in a specific industry for a while, made some connections in that time. Left my full-time job and let my contacts know I'd be doing freelance/consulting work. Landed a few starter clients from this, then just asked for word of mouth referrals (by far the best leads for me).

I'd also suggest having a landing page for your business/service, and maintaining a blog on this site where you write commentary on things relevant to your focus. Post links to your articles across social media. At the beginning and end of every post, make it known you're available as a consultant.

It's mostly just about putting yourself out there a bit. This didn't come natural for me at all, so it was a learned skill. But after a while you get kind of used to it. Routinely posting on LinkedIn et al. when you're looking for more work (phrase it like "I have room for a new client in my consulting portfolio..." or something along those lines, see who reaches out).

Just a word to the wise, if you're new to the freelance game, you'll probably accidentally undercharge all your clients for your first year (or few years). Don't be afraid to raise prices on them when you recognize what you've done.

Take your hourly rate from a full-time position and basically double it.