Read your words aloud. Read to yourself, but preferably to anyone who will listen. Read to a lonely neighbor. Read to your dog.
If possible, read for about 30 minutes at a time, but any amount of time will help.
As you read, notice the sections that cause you to hesitate or stumble–those spots need editing. Notice the sections where you need to speak more loudly or dramatically alter your inflection to make your point—and remember that your reader will not have your voice to help. Those spots may need editing.
Pay close attention and you will also catch typos and gaps in logic and the story.
After you revise, read the new section aloud again.
Read your entire book aloud.
This is the kind of time it takes to write a truly good book.
A professional editor can help you. Most of us can make a bad book mediocre, or a mediocre book more readable. Some of us can make a good book sparkle. Great editors have made flawed books into great literature.
But before you pay for professional editing, read your words aloud.