Hi Josie, Claire and Neil have given some very relevant inputs. I would just add to that the bit about Audience defining. We have to be very clear on who are we targetting via FB/ Instagram and the Ad Manager platform allows fairly tight TG selection by geography, age, gender, attributes which will all help in making sure that you get the right cost and reach metrics. This bit is also iterative and over time you will find certain audience buckets are ending up giving you more traffic/ page views and leads…or whatever else is the metric you are defining.
Hope that helps!