How would you make this sentence into a subject pronoun the woman carefully wrote letter?

The sentence "The woman carefully wrote a letter" already contains a subject pronoun, "the woman".

If you're asking how to make the sentence *about* the woman, you could use a pronoun to refer to her as the subject:

* She carefully wrote a letter.

Or, you could use a pronoun to refer to her as the object of the sentence:

* The letter was carefully written by her.