Press firm tofu for 10 minutes by placing between two chopping boards and placing a heavy book or pan on top to get rid of water. Or wrap it up with kitchen paper and put it into the microwave with 600W for one to two minutes. Remove water from the tofu completely with kitchen paper.
Cut the tofu into one inch square pieces. Place them on a flat plate. Use a kitchen paper and get rid of water completely from the surface.
Add soy sauce, ginger and Japanese sake. Mix them up by hand or a spatula roughly. Cover them up with a wrapping film on top, like a duvet cover. Set aside.