You will need 2 large baking trays, each at least 9×13-inches (no need to oil them).
Prepare the kale by removing the center rib. (This is easy to do if you fold the leaf in half at the rib (length-wise), then use a paring knife to cut away the rib and stem. Discard.)
Wash the kale thoroughly by submerging the leaves in a large bowl of cold water and gently moving them around for a few seconds to loosen any debris. Transfer kale to a large colander for a final rinse and draining.
Spread the leaves out on clean kitchen towels to dry completely before proceeding. (I change towels and flip the leaves to make sure they are completely dry.)
At this point, if you would like to store the kale for later use, roll loosely in a dry kitchen towel and place in a large food storage bag. Store in the fridge for up to 1 week.
Once kale is completely dry, chop or tear the leaves into the size you desire (I like larger pieces of about 3″x3″, but whatever you like is fine.)
Place the torn leaves into a large bowl; add the rosemary, sea salt, and drizzle with olive oil.
Gently toss the leaves by hand, making sure each piece is oiled (you may need to rub the oil onto some pieces to insure complete coverage).
Place the kale pieces onto baking trays, being careful to give the pieces enough room so they do not overlap (they will cook together and will not become as crisp as you want them to if they are touching during baking).
Bake for 9 minutes, flip, then bake 4 minutes more. Remove the chips from the oven and transfer to a platter to cool.