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Key-Squares in Pawn Endgames

In an instructional video, I have found this position:

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The instructor (Dr Muller) says about key squares that are marked with green. I am unable to get the idea behind how Dr Muller determined these squares as Key Squares. I have practiced it many times in Fritz but fail to understand the exact technique that I should employ. The instruction goes rather fast and not helpful for beginners.

Please help me to proceed as white from here.