Work out how long your writing would be if it was a straight line. The easiest way to explain what I mean is to imagine if it was written with a piece of string that you could then pull staight and measure.
After you've done that, make an MC that's basically a dot. Line up a lot of instances of these dots the length in pixels that you worked out your bit of string would be. Probably best if you use duplicateMovieClip to create these dots. Tween these dots to cross your screen.
Then create a motion guide for the tween that your line will follow. This I'm sure you've guessed should be in the shape of your writing, so when these dots cross the screen they appear to be writing the word.
The last thing to do is place a mask layer over the layer with the dots, hiding the line before it starts to form the word. This way you won't see any part of the line unless it's forming the word. (Otherwise it will look like a line following a motion guide).