This is a tough one, there are a million ways to get from A to Z. I have a system I use that I find works for me. I begin with a list of what they want to accomplish, what they like about exercise, and what they don’t. After collecting that info, I put it to the side and to a fitness assessment and from there I ask myself, what are their weak areas (ie posture, form, weight, endurance, etc.). Then I take all that info and try to design a program that addresses their goals and the major weakness I see, I try to stay with what they like (if I need to put them through stuff they don’t like I make it fun and make sure they understand why exactly they are doing it). From there I progress increasing intensity, reps, or time in various ways. It ends up being a good amount of circuit training, or specific lifts to focus on specific weakness.
Hope that helps