Drain the soaked kidney beans and rinse well. Place them in a large pot with 800 ml water and 0.5 tsp of the salt. Bring to a boil, then reduce heat to low, cover, and simmer for 40-50 minutes until the beans are completely tender and easily mashed between your fingers. If using a pressure cooker, cook for 15-20 minutes after the first whistle. Reserve the cooking liquid.