First, pour three cups of water into the empty slow cooker and use a ruler to measure the depth. Note this measurement, as this is the depth your milk will be when it is finished.
Pour water out and place the milk in the slow cooker. Heat on high for 18-20 hours with the lid off, unless you notice scorching, then switch it to low. Remove the skin that forms on the top periodically, especially toward the end of cooking, but don't scrape up any milk that cooks onto the sides.
When the evaporated milk has reached the desired depth, pour into a clean jar or pitcher and use in your favorite recipes. Store in the refrigerator for up to one week.
Notes
To clean the slow cooker, simply fill the slow cooker insert with water and let sit for 6-8 hours, then scrape off any baked on gunk with a spoon and polish it smooth with a bit of baking soda. If it needs to be resoaked (which was regularly the case with my pots and pans when I did this on the stovetop, but not yet once with a slow cooker), place white vinegar and baking soda together in the insert to form a paste. Let foam, then dab the paste thickly onto the stuck on food. Let sit for 1-2 hours, then refill with water and let sit another 4-6 hours. Rinse off and wash as normal.