For Plasma the easiest method is to centre click on the battery icon in the right hand side of the taskbar. This will disable suspend until you reboot.
You may need to click on the small ^ arrow to show hidden icons if you are on a desktop, or a laptop with a charger fitted.
By far the best method to use when installing.
Edit: This is to be done directly after the computer has booted into the live environment and before session inactivity has timed out and presented you with a lock screen. Just keep an eye on what is going on and don't go off to make a cuppa 😉