This is the third MacMini that I have done some kind of upgrade. The first two were circa 2012. One thing I learned after the first one is to make sure I kept the little tiny screws (LTS) in the right place and segregated for each step of the process. So, what I do now is print the image (from this guide) of the step which requires removal of the LTS and as I remove each one I tape the LTS with a piece of Scotch tape on to the page with the image where it needs to be reattached. It makes it so much easier to reassemble the unit. The time required as listed is a bit understated. I just finished upgrading the memory chips on the 2018 MacMini and it took me one hour and 10 minutes from start to finish. Hope this tip helps.