Not only "how do you make them tender", but how do you get enough to make a meal? Do you just save them & freeze them over a period of time from whole chickens? Around here, chicken livers are all you'll find. Once in a blue moon, usually around the holidays, there'll be packages of mixed giblets for sale for gravy making.
Connie, I don't like liver.
I don't do anything to them. To answer Breezy also, They sell packages of hearts by them self. There is a about 30+ hearts to a package. Cheap too, about $1.50. They sell the gizzards separately also. Pretty much anytime you want them. There are a few times that I haven't found them.
I just wash them off in cold water, throw them in flour and fry them only long enough for the outer crust to get a beige color. If you cook them too long, they will get tougher, same with the gizzards.