Dedicating the end of a year to cleaning up things, particularly on computers, is a good plan. I don't always succeed fully, but at least make the effort. I go through my passwords and update some of them, go through my Mail program and delete or archive much of its contents, review memberships and subscriptions I've had in the past 2-3 years and make a plan to not renew or unsubscribe, etc. I did a little bit of research into the Mac clean up project and found a good, simple article with a bit of information on the Mac OS that I had not realized: Favorite Mac Clean Up Tips - for the most recent Sierra & High Sierra, this was good information:
"In macOS Sierra, there's another way to delete disk images. Go to the Apple menu and select About This Mac, then select Storage and click on the Manage button. In the Reduce Clutter section, click on the Review Filesbutton. In the main window, click on Downloads and the click on the Kindheader to sort by that particular type. To delete a disk image. click on it, and then click on the X button that appears."
Of course, that doesn't mean it'll all get done, but it's a good plan for December & January.