Some help needed regarding how to conduct an external validation study

Hello everyone. Hope you’re well.

I’m a second year surgical resident from Pakistan (with an extremely poor background in medical statistics and a below average background in medical research), and I’m hoping to conduct an external validation of a 2016 study conducted in France. I’ve linked the study below.

This was them developing the score. I want to conduct an external validation of it.

So my initial questions are:

  1. What sample size should I take for my study to get a good result? (and how would I go about this?)
  2. What tests do I need to conduct for this external validation?
  3. How do I run those tests? (all I’m familiar with is entering data in SPSS and running some very basic cross-tab stuff)

Any help here would be greatly appreciated. Also, please do ask if there’s any questions. (I also have a PDF of the original paper)