The timing and methods you use depend on the kind of kale you are growing. You could be growing plants for baby greens or mature leaves, these two take different timings.
Take hold of the stem of a mature leaf plant right at the base and pull the main stalk. Pull it down and out until it breaks but be gentle not to break the stem. Repeat the process until you have enough kales for your use.
If you grow baby kales for salad, they will be ready in a few days. Harvest them by pinching them individual leaves with your fingers at the base of the stem.
Many people who grow kale dislike eating it maturely because it is crunchy and dry. What they don’t know us they can make it softer by massaging it before cooking it to make it more soft and tasty.