Place the collard greens in a 9x13 baking dish or large rectangular food storage container. Fill the dish or container with cold water, and let the greens sit for a few minutes to remove any dirt. Use your hands to swish the greens in the water. Drain the container, rinse the greens, and dry them with a towel.
Place the greens on a cutting board, and fold each one in half over the stem running through the middle. Use your knife to make a lengthwise cut parallel to the stem to slice the leaves off of the stem. Discard the stems.
Stack the leaves on top of each other, then roll them up. Cut the leaves perpendicular to the roll into thin strips. For a large bunch of collard greens, I usually do two stacks, since one stack would be too big.
Use the greens in recipes right away, or store the sliced greens in an airtight container in the fridge for up to 3 days.