You want to pick them both when the flower heads look full and packed, but before they begin to show any yellow flowers. Basically, the part of the plants you eat is a big cluster of buds of small flowers that will open eventually. I generally harvest mine when the heads are 4 to 8 inches across. It depends on the variety. Just cut off the head because if you leave the plant, smaller heads should form in the leaf axils for later harvests."