Proofreading is typically the final editing phase, after developmental edits, copy, and line edits. Often readers will not recommend books if there are numerous mistakes, so investing in a round of proofreading pays off with better reviews.
In this stage, we focus on typos, misspellings, missing punctuation, duplicated words, etc. In most instances, the Chicago Manual of Style is followed, although consistency is the most important factor.
Proofreading is done in Microsoft Word, using Track Changes. It's then a simple matter of "accepting" or "rejecting" the changes, once returned to the author. The time required for proofreading varies based on the length of the manuscript.