Squid - like octopus and cuttlefish - is a cephalod, and like its relatives requires careful cooking in order to ensure it is not unpleasantly chewy. At its best, it will be as fine as lobster. There are two secrets of success: one is cooking them for a very short time, the other is cooking them for a very long time. The middle road is the route to toughness.
Cut open the squid tubes, score them with a knife and char-grill or stir-fry briefly. Whether cooking Mediterranean-style or Chinese, chilli is a favourite accompaniment. Large squid can be stuffed and baked (a long-cooking method). Alternatively, slice into rings and coat with breadcrumbs or tempura batter before brief deep-frying. Thin slices of raw squid are used for sushi.