Correct file structure etc to edit themes and Magento 2 core files

Hi All,

Can someone tell me what the correct procedure is (file structure etc) when it comes to editing a Magento 2 theme and/or the Magento 2 core files.

Thanks in advance

This one is a little tricky to explain and the solution has a number of variables. Can you tell me what theme you’re using and an example of what you want to achieve?