With Justify and Strengthen questions (as well as with any question type that involves a stimulus with an argument!) it is first important to fully analyze the argument. Identify the main conclusion, and be specific! I always underline the exact part of the argument that is the main conclusion and it's a habit I encourage you to adopt as well. Then, really think about how
the premises support that conclusion. Remember that all LSAT authors believe that their arguments are solid--they believe the conclusions that they have given you and they believe that the premises stated prove that conclusion to be true. But most arguments on the LSAT are flawed, meaning that they have gaps, missing links, incorrect logic, etc. such that the premises given do not fully prove the conclusion as stated. And you need to be able to identify these logical gaps and weaknesses in arguments. So, again, the first step is just to fully analyze the argument by asking yourself: 1) what, specifically, is the main conclusion?; 2) how, specifically, do the premises support that conclusion?; and 3) why, specifically, do the premises not fully prove that conclusion?
With Strengthen questions, then, you are just looking for any new information in the answer choices that strengthens that conclusion in any way. The answer choice doesn't have to prove the conclusion, it just needs to help it out more than the other answer choices.
In Justify questions, you need to prove that conclusion 100%. For these, you can use the Justify Formula:
Premises + Answer Choice = Conclusion
If the answer choice is correct, then when you add it to the premises, it should prove that conclusion 100%.
Here are some additional resources on Justify questions that I encourage you to check out:
https://blog.powerscore.com/lsat/unders ... questions/
https://blog.powerscore.com/lsat/lsat-p ... questions/
Powerscore LSAT Podcast, episodes 11 and 12: https://www.powerscore.com/lsat/podcast/
Hope this helps!