Final answer:
The comma should be placed before 'one' in the sentence, adhering to the rule of separating an introductory phrase from the main clause.
Step-by-step explanation:
The comma in the sentence 'In order to be successful one must learn to write well' belongs before 'one' because we have an introductory infinitive phrase that sets the stage for the main clause of the sentence. Following the rule that you should place a comma between the end of an introductory phrase and the beginning of the subject, especially if the phrase is longer than four words, this sentence requires a comma to separate the introductory phrase from the main clause. Additionally, omitting the comma might make the sentence harder to read and potentially obscure the meaning, contradicting the advice against creating choppy sentences with misplaced or omitted commas.