Pour up some water in it and put it on for a couple of hours.
I’ve found that the least time-consuming way to clean your slow cooker is to fill it with it all the way up with water and crank up the heat. Then I just leave it on for some time! It’s not different from cooking food in it, and won’t hurt the machine. Read a book or something while you wait, it’s good for you!
If water doesn’t help – add baking soda and dish soap!
If the filth is very persistent, don’t lose faith! Fill up the pot with water once again, this time you add a pinch of baking soda (typically 2-3 tablespoons) and some drops of dish soap. Put your slow cooker to low and wait a few hours for the magic to happen. After some time you can pour out the water and clean the inside with a non-scratch pad and a little more baking soda. Any stains or old food residues should come right off!
White film on the pots inside? Use vinegar to get rid of it!
After using your slow cooker for some time, a white film can develop on the inside. It’s not hazardous in any way, neither does it affects your cooking. However, it can be annoying and you probably want to get rid of it. Fill the pot with water and add one cup of white vinegar to it. Leave it with the water and vinegar for 5-10 hours, why not overnight, then empty it and wash as usual. The white film should now at least be a lot less visible, if not completely removed.