The Göttingen Septuagint is the premier critical-scholarly edition of the Septuagint. If you’ve ever seen or used it, you know it can be intimidating. Fortunately, Abram K-J (Words on the Word blog) has created a primer in two parts on how to read and understand the Göttingen Septuagint. Here are the links (highly recommended):
How to Read and Understand the Göttingen Septuagint: A Short Primer, part 1
How to Read and Understand the Göttingen Septuagint: A Short Primer, part 2 (the apparatus)
And it goes without saying you should have the digital version of the Göttingen Septuagint.