With all due respect, this seemingly simple question has a complex answer. Much akin to "How do I drive a car?"
The most basic overview I can come up with goes like this:
1. Decide on a niche
There are a million freelance website designers. What makes you different and how can you offer something to your client that other may not be able to?
2. Connect with those who need your services to build a portfolio
Reach out to non-profits or small businesses to get started. You will need a portfolio and client recommendations as you grow. By getting this done early, you can use them to get larger, better paying clients
3. Build your network by offering valuable advice for those who may not know they need you
Use a platform like LinkedIn or Facebook to educate small business owners on the benefits of having a well-designed website. Offer tips on how to manage a website redesign, what to look for in a designer, how to manage a web project, etc. This plants the seed in the mind of your next customer and positions you in a place of authority
4. Have a process and system for growing your freelance business
Communication and time management are two areas freelancers of any industry struggle with. Having a process in place early will help you effectively grow without dropping the ball on the many projects you may have going at the same time.
Again, this is a very (very) high level overview. There are also considerations for pricing, demonstrating value (vs competing on price), lead generation and nurturing, outsourcing, and moving from project-based work to retainer work.
I would be happy to discuss this more with you, if you'd like. Drop me a note.
Best of luck!