It happens once you have a draft you are confident in as a whole. In this process, you are going to look for the details that may have slipped by you during the writing process. Spelling errors are often caught by spellcheck but do not trust this tool to catch everything. Word usage is also a common problem to catch in editing. Is there a word you use repetitively? Or did you write there when you meant their? Details like this seem small on an individual basis, but as they pile up they can distract your reader.