Flow materrers, in my experience, light not so much, just place them where you want them, see if they open up nice and pretty, if not move them, I have ad one who likes to plant itself to the bottom of the aquarium and one who likes the crevices of LR. Either way, it will be somewhere they get decent flow (not strong) and can get food (spot feeding eliminates that though). Just find a spot where they open is my best opinion.
EDIT: Wherever you place them they will attach with a very strong secretion, make sure if its on a rock its a rock you may be able to move later, they will hold fast to whatever it is they touch when they find somewhere they like.