First of all we have recently tried something that has been an awesome tool to teach dictation to my children so I wanted to share. First of all start with a really small sentence, especially if you are starting this when they in first or second grade, the older kiddos who need practice can handle a longer, more interesting sentence. But assuming you are starting this when they are really young then choose an easy, short sentence - literally like three or four words and make a flashcard for every word, every punctuation. So if your first sentence is 'The cat sat', then you would have one card for The, one for cat, one for sat, and one for the period. Say the sentence three times and then have them repeat it back once to you. Mess up the cards and have them put them in correct order, over an over again. (The longer selections will be more interesting and able to do this for longer). Several times through the day have them take the cards out and put them in order. The next day say the sentence three times, have them say it once, and then write it. Then day three do a new sentence and repeat the steps. On day five repeat both sentences three times each and have them write them. After a while you will no longer need to do the repetitive stuff, but it was too much of a jump for my kiddos to go from copying straight to dictation, we needed to bridge the gap.
Yesterday I made them encouraging signs - that even in their weakness, His strength would show through. Today I am going to do their chores for them. :) they will be VERY happy about that :)