From our experience, the Hugelkultur method is the easiest and most cost-efficient method to use. Hugelkultur is a method of layering organic materials such as rotted hay, branches, plant waste, and compost to the soil. You can easily fill a raised garden bed using materials lying around in your yard. As the organic matter decomposes, it breaks down into nutrient rich soil that mimics the natural soil found in a forest. You can refer to this article for more detailed information.