Harvest:
Harvest leaves from low on the stem first and work your way up the stalk. Pick leaves from the outside of the plant and work inward. Be careful not to damage the stem where new leaves emerge. Leave at least four leaves at the top of the plant (the growing crown), allowing the plant to grow new leaves for future harvest.

For more information on growing Collards, check out this great guide from Harvest To Table:
https://harvesttotable.com/how_to_grow_collards/