What are your MUST do's before feeling like your build is 100% set up?

Once all hardware and physical related things are 100% done, Windows installed and you're looking at a fresh, neat new build. What are your must do's inside the PC itself to be 100%?

I was just reading about RAM XMP, fan curves and these things. I'm a noob and I just fully finished the physical part + windows and wanna make sure I'm now also covering all bases on the other side as well.