The easiest way to get started is to turn your current employers into clients. Talk to your boss about moving to freelance and tell them you'll be working from home. You'll have to make sure to keep your rate high enough (it will look higher than your current salary, but won't be).

Pitch well and they'll usually go for it as it's cheaper to have a contractor working than a full-time employee.

I've seen this done successfully multiple times, and never seen any employer turn it down.