Core the cabbage: using a pairing knife, cut out the core of the cabbage, about 1 1/2-2 inches (4-5cm) deep. Make sure to cut it out wide enough so that the leaves are not attached to the core.
Cook the cabbage: In a large pot, bring a large amount of water to a boil (enough to cover the cabbage). Add the cabbage, core-side down, cook for 10 minutes, then turn it over (it should be core-side up now) and cook for a further 5 minutes.
Peel the leaves off one by one using tongs while the cabbage is still in water, stack the leaves on a plate. Grab the cabbage with tongs by the stem, trying to not damage the leaves. The point here is not to cook the cabbage but to separate the leaves and soften them. They should not be completely soft (fully cooked) but only pliable. If the leaves are not separating try to separate them from the core with a knife (if you've cut out too little). If your cabbage leaves are still not soft enough to be easily pliable you can add them to the pot and cook them a little bit longer. Don't overcook the cabbage or the leaves will tear.
You will need about 16 leaves.
Alternatively: you can core the cabbage, wrap it in plastic foil then freeze it whole. After it has been thawed you can peel the leaves easily and use them right away. It will take overnight to freeze it then overnight + a couple of hours on the counter to thaw it.