I shouldn't need to tell you this, but here goes: always use a spell checker. Most word processing programs, like Microsoft Word, include a built-in spell checker. Some browsers, such as Firefox, also include one.
Of course, using a spell checker isn't enough. A spell checker won't catch "too" when you meant "to." It won't flag "Connected" when you meant "Connecticut," or the contraction "it's" when you meant the possessive "its." It catches only those words it's unable to find in its dictionary. So using the spell checker is only the first step. You must follow it up by carefully reading over your text, using your own eyes and brain to try and catch any errors that remain. More on that next week.
Behind every spell checker is a dictionary. But this dictionary doesn't come preloaded with every word you'll ever use. It starts with only a basic vocabulary; it needs to learn any special or unusual words that you use.
Fortunately, you can grow your spell checker's dictionary. Proper nouns, foreign words, and new or uncommon words are likely to be missing from your spell checker's dictionary until you add them. So, if your spell checker flags a word that you know is spelled correctly, add it to the dictionary — especially if it's a word you expect to use often. Any good spell checker gives you the option of adding a word to its dictionary, so that — as Pete Townshend might say — it won't get flagged again.
The ability to add words to your spell checker's dictionary isn't just convenient. It's also important. If your spell checker keeps on flagging words that aren't wrong, over and over, this can actually make it harder for your tired eyes to spot the flagged words that you do need to correct. Our eyes and brains are imperfect. They tend to get fatigued and take unconscious shortcuts. Having lots of extraneous false positives staring us in the face makes us prone to glide over and miss the real errors, even when they've got red underlines.
Ideally, virtually every word your spell checker flags should be a legitimate error that you need to fix. Over time, as you add words to your dictionary, you'll get closer and closer to this ideal.
So use your spell checker. And don't just run it to give your work a quick once-over. Really use it as the adaptable tool it is.
It's your first line of defense against looking unprofessional.