2015 Civic SI Spark Plug and O2 Sensor Change

DIY Spark plug and O2 Sensor change for my 2015 Civic SI Sedan.
I'm sure this will work for all the 9th Gen 12-15 Civic SI, regular Civic Models may vary.
-Just his 100k miles so wanted to change spark plugs
-Highway MPG dropped from 30-32 down to 24-25 MPG and a lot of stuff I've seen online said might be due to bad O2 Sensor so I decided to change it
-Months later, still getting 24-25 MPG so still trying to diagnose what's wrong, recently cleaned MAF sensor so well see if it goes back up to +30 MPG