Unless you have an extremely large pan, plan to cook the collards in two batches. They will cook down significantly, though not as much as spinach. Heat half of the oil in a large (12 inches or wider), deep frying pan over medium heat. Add half of the garlic and cook, stirring, for about 30 seconds, until fragrant. Add half of the collards and turn them with tongs to coat with the garlicky oil. (You may need to add the greens in batches and let the first batch wilt before adding more.) Stir in half of the red pepper flakes and 1/4 teaspoon of the salt. Cook greens, turning them occasionally, for 3–4 minutes, until wilted. Transfer to a serving bowl and repeat with remaining oil, garlic, greens, red pepper flakes, and salt. Serve hot or at room temperature.