Inspect the head of the broccoli plant. When it's ready to be picked, it should be firm and completely developed. It should measure at least 4 to 7 inches in diameter.
Look at the florets around the outside of the head. They should be approximately the size of a match head.
Check the color of the top parts -- the treetops -- of the broccoli head. When they have turned a bluish-green, and are still tightly closed, the broccoli is ready to be harvested.
Grasp the head of the broccoli plant with one hand. Reach down to the stem, approximately 6 inches below, with a sharp knife. Cut the stem to harvest the head.