Mix plants with trailing, spiky upright, and "fluffy" growth habits, as well as large, medium, and small leaves. Choose a color scheme or color combinations that complement your home or landscape. Red, yellow, orange, bright pink, and white look good from a distance, while blue, purple, and dark green show best at close range.
Choose containers that fit your decor and available space and are at least 8 inches wide and deep. Be sure they have drainage holes or plan to drill your own. If mounting under a window, use a box that is a couple of inches smaller than the width of the window for best appearance.
Purchase a sterile potting mix containing peat, perlite, and other ingredients that improve drainage, aeration, fertility, and water-holding capacity. Consider using a water-absorbing polymer to decrease watering frequency. Fill your window box about half full with the potting mix, and add water to moisten the mix if it's dry. Do not use regular garden soil.
Plan to set plants about 2 to 5 inches apart in the box, depending on their mature size. Slip plants out of their pots without pulling on the stems and gently untangle any circling roots. Set the tallest plants, such as geraniums, in the back of the box. Let the trailing plants, such as lobelia, hang over the front and sides. Fill in with the fluffy plants, such as pansies or impatiens. Fill the spaces between plants with soil mix, tapping gently. Water thoroughly to settle the soil.
Window boxes require frequent watering — often daily in hot, dry weather. Soak the soil completely at each watering. Use a water-soluble flowering plant fertilizer dissolved at one-quarter strength once a week or according to package instructions. Trim dead flowers and straggly growth and replace plants that perish or look ratty. Remove some plants if the box becomes too crowded or requires watering too frequently.