You can watch the following video. There are also text instructions below the video. (including getting sddm working)
I started off with just a boot install of CentOS 8, so I wouldn’t have Gnome or any other unnecessary cruft installed. After installation, I enabled the EPEL repository – latest directions for that are here.
After that, I had to install KDE by typing:
sudo dnf groupinstall “KDE Plasma Workspaces”
systemctl set-default graphical.target
Then startx started Plasma.
I didn’t spend time getting the display manager working, but it’s pretty easy. After having done all of the above (but before you use startx), type:
systemctl enable sddm
A reboot later you’ll have your graphical login.