Look, no-one is ever going to guarantee you a 1st. Every instructor has different requirements, and what satisfies one will not be sufficient for another. Writers aren't psychic and they don't know what your course has covered, nor do they know what specific predilections your instructors have. Marking is a highly personalised business. However, you can help yourself in the following ways by telling your writer:

--About all the requirements your instructor has given for completing the assignment. You'd be surprised how many people don't bother to do this immediately, instead waiting until it becomes a problem.

-- About things your instructor deems important. Do they think Milton Friedman is a god? Please let me know. Do they lick the shoes of Keynes? I can work with that, but I have to know about it.

-- What's your feedback on previous assignments? What were the characteristics of projects on which you did well as opposed to those on which you performed poorly? Sometimes in order to mark you up the instructor isn't looking for astounding factual information, but for excellent basic research capabilities. Learn something about this.

In short, you need to learn what it is your instructor deems worthy of a first, and then communicate that to your writer. Like I said, there are no guarantees, but you can improve your prospects by speaking openly and honestly with the writer.