The best way to soften cabbage is to give it enough time to cook. The cooking times in this recipe worked perfectly for me, but everyone's stove, pans, and ingredients can vary slightly, so be ready to add a few extra minutes to let your cabbage get soft if you need to.  Now, if you're talking about sauerkraut, that's a different story. Thoroughly mix your sliced cabbage with approximately two teaspoons of non-iodized salt and let it sit overnight, and that should soften it up enough to pack it in good and tight in a glass jar for fermenting.