Poppy seed heads are ready when they are browned & dry. The photo above shows one that is perhaps even too far along, but you get the idea. You can also pick them when they are between green & brown & let them dry indoors so that you don't loose the seeds
Poppy seeds are ready when the pod has fully browned, cut the stalks & pod off, careful, each one contains many many seeds. You can leave pods on paper to dry, and then leave in a dark & dry container until Spring!
These pods were left a little too long, and there is not many seeds in them, most of them spread by the wind into the garden!