Gather all of your supplies. When choosing the flowers to plant, choose a variety of plants including some that are taller, like geraniums, some that are trailing, like lobelia, and flowers that are more full, like pansies and impatiens.
Make sure your window box has drainage holes for water. If it doesn't, poke a few holes in the bottom of your window box so your plants don't get soggy feet.
If your window box is not already attached to your window sill, attach it using nails or screws.
Fill the window box about half full with potting mix, and add water. Gently turn your plants upside-down in their containers and tap them out. Holding them by their root ball and being careful not to harm the stems, place the flowers in an arrangement that you like. Try to plant the taller flowers closest to the window, with the fluffier flowers in the middle, and the trailing plants on the side furthest from the window.
Use a trowel or your hands to add potting soil to fill in the holes between the plant. Gently pat down the soil around the bottom of the stems to make sure the plants are secure.
Water the window box generously and enjoy your new window box.