When teaching a child with Dyslexia how to use a computer to write just follow these steps, and you'd be amazed how quickly your child will become a confident writer and child in general.

Make the whole exercise a fun experience for your child. Have them watch the magic of a computer spell-check , write out a paragraph yourself, making some common spelling mistakes, explain You often spell this word a way the computer doesn't like. Spell the words almost correctly, maybe transposing a couple of letters show the child how computer underlines the errors, talk about how magic it is to find the words it doesn't like. (sometimes computers don't like a correctly spelt word)

First copy the wrongly spelt word so you can paste in, then right-click on the spelling error and Word will probably give you a few choices of what the word should be, but choose Auto-Correct then Auto Correct Options, put the incorrect spelling along with the correct spelling in and click ok. Do this with the other spelling errors, and then go back to your paragraph click right after the wrong spelt word and hit the spacebar... and like magic the word with auto correct.

Talk about how cool it is, and try typing the word incorrectly again, magic it fixes itself. You will probably know some of the words your child has a problem with, make sure a couple of those words are in the paragraph you write.

Ask them to write something using the word in a different way. Watch the magic again, get excited with the child.

Avoid saying "you spelt it wrong" just make the computer finding mistakes a fun thing, saying things like, "oh look it's found a new word, come on let's make the computer change it."

Rather than "lets make the computer do it right or fix it"

Depending on the ages of the child, get them to write something to their level perhaps only a sentence, then program the spell-check with anything it finds. Do another sentence and so on, until you have found as many of the words commonly used by the child as possible.

Let your child write something by themselves.

Come back, and do the spell-check thing. Soon you will only have to check occasionally, or when they ask you to "do the Spell-check magic"