Don't use the "pronoun header." A pronoun needs an "antecedent" (a noun within its own sentence to refer back to), so you should not use the pronoun as a first word in a sentence...because, as the first word, there is no in-sentence antecedent noun!
The most common pronoun header is "it," but there are many others. Additionally, a pattern of "pronoun header" sentences flattens your writing and makes the writing less rich. Examine the first word of each of your sentences to see whether that first word is a pronoun. If the sentence begins with a pronoun, then rewrite it.
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