Measure the area where you plan to build your raised flower bed. If the area is large, consider building more than one bed to provide access to and around it and make it easier to reach your plants.
Mow or trim all unwanted plants from the area. You needn't dig out old lawn grass or small weeds, but do remove larger weeds and rocks. Mark the corners of your bed by driving stakes into the soil and tie string from stake to stake.
Lay several sheets of newspaper or flattened cardboard over the area inside your raised bed. Overlap pieces to cover the ground completely. Build a wooden frame, if you wish, using boards that are 1 inch wide by 12 inches tall. Nail or screw the boards together at their corners and then place the frame on the cardboard or newspaper.
Fill your raised flower bed by shoveling layers of organic materials, such as compost of any type, lawn trimmings, leaf litter, kitchen and yard waste, peat moss, wood ash and any other materials you have available, into it.
Rake the top of your completed bed level and then run a sprinkler over the bed for 20 to 30 minutes. You can begin planting flowers immediately.