Mark off your rectangle. Rectangles are for beginners - hold on to your curvy design ideas until you have more experience under your belt. Once you have marked off the spot you can begin clearing. You need the spot to be free of all grass, weeds, and trash. Then dig down about the depth of your shovel and turn and flip the soil over and over. Once the area is clear spread the manure and compost. Try and get the area as level as possible. After the area is level, give it a nice gentle shower with the hose.