This comes from Divine Caroline. She has an actual recipe, but I'm just going with a descriptive technique. It is so easy, and the results are so delicious. Go get apples and make this now.
Get apples, say 12-16 of them, or more, up to however many will fit into your crockpot. It's absolutely okay if they're a bit past their prime, have a few bruises, etc. Peel them*, core them, and chop them into smallish pieces. Dump them into your crockpot. Pour in apple cider, to about 2/3 of the way up the apples. Add spices (for 12-16 apples: 1 tsp cinnamon, ¼ tsp nutmeg, ¼ tsp allspice, ¼ tsp cloves, and ¼ tsp salt). You can add sugar -- the original recipe called for about 1 T brown sugar per apple used -- but you might not need it. If you use mostly sweet "eating" apples, you probably won't need to add sugar, but if you use tart "baking" apples, you might need to sweeten it up a little. Go by your personal taste, and start tasting once you're more than halfway done.
Cover crockpot, turn to high, and bring everything to a bubble. Turn the heat down to low, stick a wooden spoon or a couple of chopsticks into the pot to keep the lid from sealing tight, and cook all day (at least 6-8 hours), stirring a few times each hour, until it's got a nice dark color, the apples have turned to mush, and the liquid has cooked off enough that you've got a nice thickened consistency. You can get in there with an immersion blender towards the end if you want to make sure it's nice and smooth. Store in the refrigerator or freezer, or can for longer storage.
* Note: I've recently read that it's better to use unpeeled apples, because the pectin in the skins will help the butter to thicken. Keeping the peels on certainly saves time, and adds fiber and other nutritional goodies. I will definitely try this the next time I make apple butter!! (If you try it, let me know how it turns out!)