Depending on location, ie. the amount of direct sun, baskets will need to be watered anywhere from daily to every five days or so. Incorporated into the potting soil is Osmocote, a 13-13-13 fertilizer that will last through mid summer. After that a bi-weekly feeding of soluble fertilizer will be required. 20-10-20 or a similar formulation will work well.
Besides watering and feeding you will want to do a bit of pruning to maintain shape and control unwanted stretching. Deadheading is one form of pruning whereby you remove the spent blooms by pinching the stem just below the flower. Some varieties of petunias like the “Million Bells” types are self pruning. Impatiens also shed their blossoms freely and continue to bloom. By pinching the growing tips of any plant you will control outward growth and promote branching back along the stem which keeps the basket mounded, full, and tidy
Going away for a week or so in the summer? Set them along a north facing wall of your house and water well. They will easily last the week.