Place a sieve over a bowl, add glutinous rice and short-grain rice, and rinse them with water. Repeat this process several times, changing the water each time.
Soak the rinsed rice in a bowl of water for at least 1 hour. Then, drain it well and mix it with mirin and soy sauce.
Rinse the inside of the cleaned squid under running water. Stuff each squid with the rice, filling it about one-third full. (The rice will expand as it cooks, so this is just the right amount.)
Secure the open end of each squid with a toothpick (or bamboo skewer), weaving it through as if sewing, to keep the rice from spilling out while cooking.
Fill a pot with plenty of water and bring it to a boil. Once boiling, add the stuffed squid and cook over medium heat for 20 minutes. Then, drain the squid and rinse the pot.
Add the seasonings (mirin, sugar, and soy sauce) and water (as specified in the recipe) to the pot and bring to a boil. Once boiling, return the squid, cover with an otoshi-buta, and simmer over medium heat for another 20 minutes. Your ikameshi is now ready.
You can enjoy it freshly made, but letting it sit in the cooking liquid for a while allows the flavors to soak in even more deeply. Serve as is, or slice into bite-sized pieces according to your preference, and enjoy.