Wash in cool water. Greens may need several changes of water to remove all the grit that gets trapped in the coarse leaves. They can be spun in a salad spinner to remove water or drained in a colander. The thick stem and vein should be removed before cooking. Fold the leaves in half at the vein and cut just along the inside of the vein, which removes both the vein and stem.
Another way to remove the vein and stem is hold the stem in one hand and run your fingers from your other hand down the stem to strip off the leaves.