I’ve done this quite a bit, with pretty much 100% success, so it’s not too difficult.
All you have to do is trace along the center of the lines of text. Basically, make an open mask that follows the exact same path that a person writing it would. Use the “stroke” plugin, but at the bottom select “Reveal Original Image”. This makes the stroke path not so much “paint” on the screen, as reveal whatever is originally on the layer. This means that if your mask is drawn over transparent areas in the layer, it won’t do anything (which is usefull for crossing T’s and dotting i’s).
Set your brush size large enough to be able to get the entire line of the letters in. An easy way of checking this is, after you’re drawn your mask and applied the stroke, set “start” and “end” properties so that the stroke reveals everything, and then simply flip the stroke on and off to see whether there are any spots that it didn’t reveal. 95% of the time, you shouldn’t have to worry about changing your brush size during the course of the reveal, but if your font changes width drastically, you might have to… but usually you can get around that by tweeking the path.
Also keep in mind that handwriting is fast enough that you usually shouldn’t need to worry about every little detail in the animation.
You can always use this path again, later, to animate a pen, if you so desire.