I've come to the conclusion that the following works best:
1. Download the installer via the Apple Store, but DON'T run it when offered the opportunity to do so (it should download to the Applications folder). Just quit it.
2. Have on-hand a USB flash drive of sufficient capacity (at least 16gb).
3. Now, use either "Install Disk Creator", "DiskMaker X", or the terminal app to create a bootable copy of the installer on the USB flashdrive (both of these apps are free)
4. Next, move the installer app from the Applications folder to somewhere else (an "archive volume", perhaps) for safekeeping. You may need it again someday.
5. Now, boot from the USB flashdrive.
6. At this point, you can either do an install "over" your existing installation, or re-initialize the drive and do a "completely fresh" install. YOUR CHOICE.
7. When done, put the USB flashdrive installer somewhere for safekeeping.