Before you start any marketing or campaign you need to be clear what you goals are.
Trying to increase sales will be different than trying to increase retention or reduce cart abandonment. A specific and measurable goal is ideal but even a rough idea of what you're wanting to accomplish is important.
Once you have a goal, it becomes easier to know what messages you want to send and who you want to send them to. That's when customer segmenting can really shine, especially when paired with the advice in Repeat Customer Insights.