Normally you’d just run sudo apt install … but in this case you are adding a new repository so you have to follow the extra steps of adding the signing key and so on first.
I’ve tried running Plasma 5 on Wayland occasionally but due to having NVIDIA card there’s always been bigger or smaller annoying issues so I always reverted back to X11.
Yeah, I tried it again this morning and got a black screen with mouse trails. I also have an Nvidia card and will give it another go when Plasma 6 comes out.