How to Dead Head Plants
Removing the spent flower heads on plants encourages the plant to produce more flowers. It also makes the plant look nicer and is the fastest way to improve the look of one's garden.
I do dead head plants in spring and early summer. Towards the end of summer and fall, however, I tend to leave the seed heads on so birds will have winter food. Many native plants are a ready source of food for wildlife including black eyed Susans, purple coneflowers, Autumn Sedum and perennial herbs.
Before snipping anything off, take a look at the plant and observe how it grows. Remove the dry seed head and whatever stem portion is dead above a growing bud. This should encourage the plant to grow in a more bushy form and produce more flowers.
One more note on removing spent plant seed heads. For years I used a variety of pruners, taking periodic breaks so that my hands didn't cramp up from all of the repetitive movement.
Now I have a couple of sewing thread snips dedicated to gardening. These work much faster and easier than pruners and I can rotate between my right and left hand.
Spent flower plant heads can be stored in bags for use next year. They can also be composted.