If a lid sticks, tap around the edge, or pry up under it, to release any vacuum pressure. It should then open easily. Start with the longest-cooking food, and when it is boiling, add more jars. Learn cooking patterns that fit your lifestyle. If you boil them actively over a long time, add more water.
Pintos take the longest.
Most grains cook better if you preheat the water in the solar cooker for an hour or so. Long grain rice can be put in the cooker in cold water. These cook in jars with little or no water, or they can be added to beans and grains. Potatoes and sweet potatoes cook well on the rack, if lightly oiled. Otherwise, put them in jars.