Well, for potato skins and beans/peas they all need to be cooked first. The raw versions are not good for them and are considered mildly toxic. Also do not feed them onions or garlic if they are layers, as the flavor will transfer to the eggs. The other thing to stay away from is rhubarb. Never feed them rhubarb.
OTHER THAN THAT
you can just chuck it in there and watch them fight over it!
Example: a couple of grapes will insight a riot in our coop.
For ours I cut them a little like for a cucumber I would cut it into three pieces if its a normal size one. This way the ones lower on the chain can have a chance to get some too. Not much in the way of preparing though.