Soak your beans/ lentils overnight in plenty of water as they are likely to triple in size whilst soaking.
Drain and rinse them under cold tap after being soaked.
Place in a slow cooker with at least 2 inches of water covering them, cook on low for about 4 hours, they should be tender, if not add a little more water and cook for further hour or two. My beans are usually cooked by lunch time
If you have a voraciously hot slow cooker you may need to add more water or shorten the time. Slow cookers do vary temperature