One of the easiest ways to fertilize is to mix in a granular fertilizer to the soil before plants are set in which slowly releases fertilizer throughout the growing season.  You can also add supplemental feedings through the use of water soluble fertilizers throughout the season for larger plants that require more than the granular can keep up with.  Otherwise make sure to add a good plant starting fertilizer immediately after planting and continue with fertilizing once a week during the first 3 weeks after planting to help the plant get established.  Throughout the season the use of an all purpose fertilizer (20-20-20) added into your watering can once a week will work best in most cases and will help plants produce lush greenery and beautiful flowers.