One of the tricks to getting tender calamari rings every time is to marinate the raw squid rings in milk or lemon juice. Marinating it will keep it from getting too chewy later. Cover the raw squid rings with milk or lemon juice and marinate for 30 minutes. If using milk, you can leave it for up to a few hours, but if you're using lemon juice, stop after 30 minutes so that the squid doesn't cook from the acid of the lemon.