Hi,

To be honest, the question is quite broad. The short answer is - in the cases with market context (Market entry, M&A, launching a new product, entering the new market) - always start with the market to understand the big picture.

Long answer - practice cases and build the right skills. Here is an algorithm how to develop this skill:

1. Make sure you understand all the different types of cases that you may face
2. Identify the most appropriate structure for each type
3. Solve 30-50 cases and try to understand where do they fit in your categorizations
4. Based on your experience think of the potential variations in structures within each of the case types

Here is some inspiration for you. Below you can find a list of the most common case types and some high-level recommendations on structuring:

1. Market sizing - structuring from the supply or demand side. Structuring using a formula or using an issue tree
2. Profitability - basic profitability framework. Remember about different revenue streams and product mix
3. Market context cases (Market Entry, New product, Acquisition, etc). Always start with the big picture "market". Finish with something specific to the case (e.g. How to enter?"). Structure it as if you are defining the work streams for the real project.
4. Operational math problem (e.g. Should we increase the speed of an elevator or just buy a second one? How should we reduce the queues? Etc.) - Structuring as a process / value chain, with inflows, operations, and outflows
5. Cost cutting - I provided the recommendations on structuring it here: https://www.preplounge.com/en/consulting-forum/operations-cases-mck-1105#a2134
6. Valuation - Purely financial structure with cash flows, growth rate, WACC / hurdle rate, etc.
7. Synergies - revenue synergies (price, qty, mix) and cost synergies (value chain).
8. Social / economics cases (e.g. How to improve the quality of life in the city? How to increase the revenues of the museum?) - huge variability. Practice 3-5 social cases before the interview

This is just for inspiration. You should ideally make your own system.

Best of luck!