Trim the thick stems and any wilted or yellow parts of the leaves form the collards. Cut the leaves crosswise into 1/2-inch strips. Wash them in plenty of cold water and drain them well. You will have about 12 cups of shredded leaves.
Heat the oil in a large deep pan over medium-high heat. Add the red pepper flakes and stir until fragrant. Add the greens and cook, tossing and stirring, until they are coated with oil and pepper and wilted but maintain their vibrant color, about 4 minutes. Transfer to a serving bowl, season with salt and serve.