Good spelling is important when you are writing for a wide audience so it is important to spell-check carefully. However, there are some errors that no software will pick up because what you have typed is a word, just not the one you needed! Here are some of the commonest errors and how to spot them.

[edit] Steps

Watch out for homophones. These are words which sound the same, but are spelled differently: commonly confused on wikiHow are:
- allowed/aloud - allowed means permitted, whereas aloud has the same meaning as out loud
- bored/board - bored means to have lost interest; a board is a flat piece of wood
- break/brake - break means to damage, but brake is what you do to slow down a car or a bicycle
- new/knew - new is the opposite of old whereas knew is the past tense of to know
- piece/peace - a piece is a part of something (it even has the word pie as its first three letters!); peace on the other hand means absence of noise or hostility
- sure/shore - sure means certain, but shore means the beach or coastline
- site/sight - site is the correct spelling when you mean the place where something sits - like a website! Sight on the other hand means something you can see.
- stairs/stares - stairs are steps; stares is a verb meaning to gaze intently
- steel/steal - steel is a metal, whereas steal means to take something that does not belong to you
- they're/their/there They're is a shorter way to say they are. Their is possessive - it's their fault. There is a place - over there.
- through/threw - through is a direction (as in straight through the door), but threw is the past tense of throw
- to/too/two - Two naughty boys who went too far went to jail.
- whether/weather - whether indicates a choice - whether or not, whereas weather means sun. rain, snow etc.
- which/witch - Which witch is the wicked witch? (apologies to all wiccans!)
- whole/hole - whole means complete whereas hole means a gap or space where something is missing
- right/write - right means correct or the opposite of left, but write means to compose (words or music)
Use apostrophes correctly:
- it's and its cause problems for many people. it's means it is whereas its is the possessive form. It is the only possessive form which does NOT have an apostrophe - hence the confusion. The dog wagged its tail.
- Other than that, use an apostrophe where one or two letters have been missed out when a word has been contracted - eg. where is becomes where's. Plurals do NOT need an apostrophe.
- Plural possessives have the apostrophes after the s. The dogs' tails wagged in excitement.
Be aware of noun and verb differences in words like:
- advice/advise advice is the noun whereas advise is the verb
- practice/practise practice is the noun and practise is the verb (in British English)
- affect/effect - affect is a verb , effect is a noun. So you talk about the effect of something, but affecting someone. Another easy way to remember this is affect = affection while effect = cause and effect.
Watch out for words that sound similar:
- dose/does
- of/off
- quiet/quite
- chose/choose
- lose/loose
- accept/except
- past/passed
Take care not to write of instead of have after modal verbs such as would, could, should, must, may and might. The confusion comes from the contraction - would have becomes would've which sounds like would of.

[edit] Tips

There are significant differences in spelling between the different varieties of English used all over the world, and that includes spelling conventions.
American English uses -ize where British English uses -ise in words like socialise/socialize. Both are correct, provided you are consistent throughout the article.
American English omits the u in words like color and honor, where British English retains it - colour and honour.
The colour is grey in British English, but gray in American English.
British English doubles the l at the end of words like travel when adding a suffix - American English does not.
Embrace the glorious variety! It could be worse - in Tudor England there were no spelling conventions and people spelled as they wished. It makes for interesting reading!