Ideally, before you hire an editor, you should know the answers to three questions:
1. When should I hire one?
2. Why am I hiring one?
3. How do I find the right one?
Writer’s Digest asked three professional editors how to answer these questions for yourself — see what they said here.
And PS: They suggest you resolve these questions well before your manuscript is ready, so you can book the perfect proofreader several months before you actually need him or her.