This article explains how to install Plasma on openSUSE in case you had initially installed a different desktop environment, or no desktop environment at all, and decided that you want to try Plasma or maybe even switch to Plasma completely.
To install Plasma you simply need to install the required patterns.
zypper install -t pattern kde kde_plasma
Logging into Plasma
After Plasma is installed, you can log into it by simply logging out and selecting Plasma under Session Type at the login manager.
Switching to SDDM
If you're switching to SDDM you may want to switch to the KDE login manager (or display manager) SDDM too.
Edit the file /etc/sysconfig/displaymanager and set DISPLAYMANAGER="sddm"
Or use YaST /etc/sysconfig/ Editor, and select SDDM under Desktop -> Display manager -> DISPLAYMANAGER