The importance of training:
CPD: Training counts
If you're interested in freelance editing, you should look into training. I really can't stress this enough.
Some publishers won't hire freelancers without training or certification. In the USA, we have the Editorial Freelancers Association; they offer online courses. Other organizations exist, but I check EFA courses first. The EFA's invaluable.
What a publisher looks for before hiring you as an editorial freelancer
Many thanks to the SfEP and PTC, who provided these links originally (barring the EFA link, of course).