Step 1: Prepare the soil. Pour potting mix into a bucket and add water until the soil feels like a damp sponge. Stir in a time-release organic fertilizer, such as Dynamite. For full-sun and shade boxes, add a moisture-retaining polymer, like Soil Moist. Cover drainage holes with coffee filters, then fill two-thirds of the box with the potting mix.
Step 2: Plant the box. Arrange plants, still in their pots, on top of the soil to approximate their positions. Gently tip them out of their containers. Start planting in the center of the box and work toward the sides, adding more soil around the roots as you go. The final soil level should be about an inch below the top of the box.
Step 3: Water lightly. Moisten the base of the plants to settle them into the soil and eliminate air pockets. Add more soil as needed. Place or hang the box beneath the window and water thoroughly until the drainage holes start to drip.