“Not to exceed manufacturer supported memory”. I don’t understand that."
It means that you can't just install any old RAM.
Do exactly what the Crucial scanner has recommended - it can be trusted.
"Is it easy for someone inexperienced to install extra RAM?"
Yes, it is.
1. Open the case.
2. fit the new RAM modules into the slots provided, and ensure that the retaining clips click into place.
The RAM modules can only be inserted one way, and you'll easily see which way that is by studying the modules and slots - there is a notch on the module that correspinds to a lug on the slot.
Replace the case, and turn the computer on - the BIOS will detect the new RAM as it does its Power On Self Test (POST), which is the bit that happens in white text on a black background before Windows loads.
You should find that upgrading the RAM makes a big difference.