Always use your computer’s spell-checker, but never rely on it alone.
(a) Generally. Get to know your computer’s spell-checker, and especially how to add and edit its custom dictionaries. Words (and names) that your computer flags over and over again -but that you know are spelled correctly- you should add to your custom dicionary.
(b) Self-check. Make a habit of checking your spelling every time you enter new text or edit existing drafts. The best routine is to save the file one last time (having saved regularly while working on the document), then run the spell-check, and finally close the file, saving changes.
(c) Proofreaders. Don’t rely solely on the computer’s spell-checker, it can’t substitute for human eyes. Invariably, the more eyes that see a draft, the better. Your work product reflects your level of professionalism, and neglectful errors suggest less-than-rigorous work habits.
(d) Autocorrection. Be careful about letting your word processor make automatic corrections such as search-and-replace. There’s a failure rate with automatic substitutions, and the practice can introduce new errors.
Gardner, Bryan, The Redbook: A Manual on Legal Style (2nd Ed.), Thompson West, 2006.