Finely slice the cabbage and place it in a large bowl.
Add sugar and salt to the cabbage. Massage the mixture vigorously for 5-10 minutes until the cabbage releases its juices.
Transfer the cabbage to a clean jar, packing it down tightly.
Mix the apple cider vinegar with boiling water and pour over the cabbage, ensuring it's fully submerged.
Cover the jar with a cloth or loose lid and let it ferment in a cool, dark place for 1-4 weeks, tasting periodically to achieve your desired level of tanginess.
Once it reaches your preferred flavor, refrigerate to halt fermentation.