Add it to just-boiled water and mix/shake actively until it cools down to room temperature.
Add water or heat up and add more soap flakes to get necessary density if needed.
Source: https://ru.wikihow.com/%D1%81%D0%B4%D0%B5%D0%BB%D0%B0%D1%82%D1%8C-%D0%B6%D0%B8%D0%B4%D0%BA%D0%BE%D0%B5-%D0%BC%D1%8B%D0%BB%D0%BE
Outer-disinfection-shower_