If you decide to self-edit, put your manuscript in a drawer for 3 months to a year. Seriously!
Come back to it with a fresh set of eyes. If you are not surprised by your own ideas, words and word combinations, then it hasn't been long enough. Back into the drawer it goes!
If you are in a hurry, then my greatest recommendation is hiring a freelance editor. Professional editors are trained. They know what to look for. They understand how something simple, such as punctuation or basic grammar, can reveal the true essence behind a sentence.
They point out discrepancies, check the nitty-gritty details, suggest organizational change for better flow and walk the fence between the readers' needs and the writer's desires. They have perspective!
They also love editing for authors because editors enjoy reading and learning new things or seeing old things in new ways. E-book editors have insights into preparing your manuscript for publication. They can assist in the technical details of conforming your manuscript to the styles of the publishing package you've chosen.
As an editor, my most common comments are ". . . for clarity and flow." I know the essence of your meaning is there. It just needs to be unveiled by a little tweaking: a comma added here, a comma deleted there; a little re-ordering, a little tightening; or perhaps a pronoun replaced by a noun; etc.
Most authors have stopped looking at their manuscript at that level a long time before they got to the 20th page. Time for a second pair of eyes!
Here's an interesting article about why it's challenging to catch our own typos.
Meet the Editor
I'm Coreen, and I am a copy editor, writer, instructor, digital marketer, and student of PR and Communications for organizations doing positive work in the world.