As soon as a flower's petals have fallen off, or once it has withered or browned, remove the entire bloom, from the base of the flower stalk. The flower stalk itself will eventually die off so if there are no new flower buds on it, remove it as well. Use scissors or hand pruners to remove tough stalks without damaging your plants. Some flowers are clustered on large scapes or long spikes - remove the entire spike once the uppermost flowers have finished blooming.
Some plants shed off their old flowers every day while other flowers last on the plant for 3 or 4 days before fading. To keep deadheading from becoming overwhelming, take a daily tour of your garden and remove any spent blooms you come across. If you have little time for maintenance, try using`Wave' Petunias, Impatiens, Annual Vinca or Periwinkle (Catharanthus roseus) and Wax Begonias. They are all annuals that don't require any deadheading whatsoever.