plug in controller
start steam, big picture
select FF8
select controller (right icon)
select suggested layout bar
switch on community layout
choose the one you like (with A on the pad)
select it and apply (with X on the pad)
exit big picture and play

(if it doesn't work, start the game in big picture, then save exit and restart without big picture)

PS: i wrote this notes because after spending an hour reading "unplug controller not usb" and so on, i was still unable to use a controller with this game...