Posting a short video or a few screen grabs will get you much better responses.
If the hand is in the shot, isolate it through keying or rotoscoping. If the shot or paper is moving, track it. Isolate the writing from the paper (likely with luma keying and masks). Create a blank paper plate to lay underneath. Animate the writing.
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Lots of ways to do this. What I've done previously was to have the letters drawn faintly on the paper - it has to be visible to the hand talent but not too harsh that the camera is able to capture the lines.
Then rehearse with the hand talent on how to write the letters. IOW, you should have a very good idea how each letter is written.
Then track the tip of the pen but ensure that you shoot at an angle where it's not obvious that the pen isn't actually inking onto the paper - a top-down POV is good.
In AE, use one of the many techniques to create the writing effect. You'll also need to create a luma matte of the hand and the pen - be mindful of highlights on the pen which will mean having to roto parts of the pen.
Since you will be blowing the text off, it is a good idea to work this into the writing on technique that you choose.
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