I haven't fed mine greens before (other than what they might peck at while they're outside). But if/when I do, I would probably only give them to the birds once a week. Probably chards or something of the like. Just expect some messy droppings with the greens! They'll return to normal within a couple days though
I give my birds mixed organic greens 3 times a week. I also give them minced carrot, minced broccoli tops and defrosted corn on alternate days. I don't give them a huge amount but enough for each bird to have a few bites. They just love their treats and wait for them every morning.
I serve them on a large plastic tray.