Wash, peel, and take out the seeds from the apples. Slice thinly. Measure out four quarts of apples. Pour all apples into slow cooker.
Add sugar, cinnamon, cloves, anise pod, salt, and lemon juice. Stir to incorporate everything.
Turn slow cooker on high for 4-5 hours. After the first hour, remove the star anise pod, if added. Every hour, stir the apples.
Once the mixture is bubbling and cooking well (after approximately 4-5 hours), turn the slow cooker to low and leave for 10-12 hours. I usually do this in the evening so it can sit on the low setting overnight.
After ten hours, start checking the slow cooker. Stir mixture using a strong wire whisk or hand potato masher to break up the apples.
Once the butter is the consistency desired, use a blender, food processor, or immersion blender to break down the rest of the apples to achieve an even consistency.
Remember that apples contain a large amount of water and will cook down considerably. They also vary a lot in how they cook. My apples had a lot of liquid in them so I needed to give them 2 extra hours on high (with the lid off) to make sure my apple butter turned out as thick as I wanted.