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. Add the plants. 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.