The object is to pick them when as many side spikes and therefore color is open. Spikes that open from the bottom up should be picked when approximately two-thirds of the spike is developed, but before the bottom flowers begin to fade. Spikes that open from the top down, would be picked just before the flowers at the top start fading. Plants that have extra moisture in them, require quick drying. Pick, hang and dry plants include Calendula, celosia, and marigold.
Some flowers practically dry on the plant, such as hydrangeas.
Roses can be picked at various stages, depending on how you plan to use them. If air drying, pick them when they are in the bud stage or just slightly open. Never pick them fully open, or they will fall apart. As with any new hobby, trial and error will give you more information. Finding the right location to dry your flowers may also take some experimentation.