Break up the flowerbed site 6 to 9 inches deep with a rototiller or pickax and shovel. Remove any debris you encounter, such as plant parts and rocks, which interfere with root development.
Spread up to 3 inches of well-rotted, aged manure over the turned flowerbed. When using store-bought manure, broadcast no more than a 2-inch layer of the organic matter over the surface. Cover the area with 2 inches of well-rotted poultry manure, or 3/4 inch of processed poultry manure.
Incorporate the manure uniformly into the ground with a shovel or rototiller. The blend is uniform when the top 6 to 9 inches of soil display the same color throughout. Rake the bed to a smooth surface, and plant your flowers.