Remove the stem from each of the collard green leaves by using a sharp knife to slice along each side of the stalk. Discard the stems, or reserve them for another use (stir fry, pickles, etc). You will not use the stems in this recipe.
Stack the leaf halves in a pile and slice the leaves into thin ribbons, about 1/4 inch thick. Set aside.
Heat the olive oil in a large skillet over medium heat. Add the crushed garlic and cook until fragrant, about 1 minute.
Add the collard greens and toss them in the garlic and oil. Cook until all the leaves are wilted to your liking. Add salt and pepper to taste and stir occasionally. Turn the heat down as necessary; cooking on too high heat will result in crispy greens. Stick to medium to medium-low heat.
If using, add the red pepper flakes close to the end of cooking. Stir to combine.
Remove from heat and serve when the greens are tender, but still bright green, about 10 minutes.