How to properly lay Tatami mats
I wouldn't know, but I did some brief poking around &amp; found some items you might find useful.

I'll summarize what I gleamed to answer your questions:


The Real (see 3rd bullet point) Tatami just goes right on the floor with nothing beneath, Tatami's are made to breathe. From an old Blog Wood Used Under Tatami Mat, scroll 2/3rd's down to 2004/8/10.
Just not fitting them tightly &amp; Etiquette of the room &amp; of the Tatami. See YouTube video Japanese Tatami Room Etiquette
I didn't run across anything about Standard or Premium. But, Standard would seem to have a synthetic foam core &amp; Real Tatami or Premium has a rice straw core. You'll have to ask the manufacturer, dealer or retailer about Standard's floor treatment or breathability.
I didn't see any tutorial, but the video in Bullet Point 1 gets into it a little &amp; there doesn't seem to be much of anything to it beyond throw them down &amp; butt them tight.
I did find something on cleaning though. See YouTube video Tatami Tips.