First, you start off with a kabocha, or butternut squash. Clean the outside thuroughly because you're going to cook it with the skin. Chop it down into small pieces, there's no rule. I prefer little triagular shapes. The, throw in some some oil in a pot, high heat and throw your kabocha into it. Stir fry it around for a bit and then when ever piece is shiny with oil, pour in a bowl of soy sauce and mirin (a 50/50 mix is good, but flavour is up to you). I actually didn't have mirin so I poured in some Gekkeikan sake that had been sitting in my fridge for too long. Add some water so that the kabocha is just sticking out of the water and cook covered for about 10 to 15 minutes depending on the size of your kabocha. You can test if their ready if you can cut a peice in half with your chop sticks (for cooking, I use thicker Chinese chopsticks so keep this in mind when your testing if they're ready).
After you decide they're done, carefully pick them out of the pot, maybe pour a bit of sauce from the pot. Kabocha can be served hot or cold, but either way, they make an awesome side dish with a hot, steaming bowl of rice.