When planting a window box that will be fixed on brackets you should use a lightweight mixture based on peat or peat substitute, else you can use any soil-based compost, as it retains moisture better than soil-less or peat-based mixes. Incorporate water-retaining granules in the compost mixture. Half-fill the box with the right compost mixture.
Choose the plants that you think will look good for you. Some people prefer a mixed planting made of some trailers and both flowering and foliage plants. No matter what you prefer, is your personal choice, but before you plant them try to make an idea about how they will look together. To help you visualize the entire arrangement, arrange the plants in the box while they are still in their pots.
When the arrangement look right to you, take the plants out of their pots and insert them in the box, firming the compost around each root-ball. Plant more closely than in beds or borders for a more compact look but avoid including plants that will overpower their neighbors. For a low maintenance arrangement you should chose plants that do not need so much water and can tolerate drought. Sun-loving flowers, grey-leaved plants and architectural plants are all good choices.
Fill the gaps with compost and firm well to remove air pockets. Water thoroughly after planting and make sure that the compost never dries out, especially the peat-based or soil-less mixtures which are difficult to re-wet. In warm weather you may need to water them daily or even more often.