The process of editing is one each writer develops on his own, through experience, trial, and error. There is no definite number of drafts you should write before you can consider the manuscript finished. There are, however, some techniques that will probably prove helpful.
First of all, if your schedule allows it, set the work aside for a few days. After the writing has had a chance to cool, errors and awkward phrases will jump out. Once you do look at the work, try to cut it. Eliminate anything that isn’t essential, as well as redundancies, irrelevancies, statements that are too obvious, unnecessary words, and circumlocutions. (Don’t worry about being too brutal; you can always put material back.)
Let the material rest again (for at least an hour or two), then read it aloud. This is probably the best way to discover awkward phrasings. If you stumble over something, fix it. Reading aloud also can tell you where you’ve cut too drastically, damaging the rhythm of the piece.
Assess the logical order of the remaining elements. Some writers use highlighters or colored pens to color-code the work’s major elements to make sure the structure best suits the point they’re trying to make. Next, check your word choices. Look for imprecise verbs and weak nouns that require too many modifiers. Finally, check for consistency of verb tense, verb agreement, punctuation errors, and misspellings.
It is possible to overedit. If, for example, you find yourself rewriting everything over and over, and seldom or never putting a manuscript in the mail, you might be using editing as a means of avoiding potential rejection. Most writers, though, are far more likely to be hurt by too little editing than by too much.