I'm hardly a master gardener but we put 1/2" grid hardware cloth on the bottoms to keep moles and voles out. Our beds are around 16"-18" tall so I lined the bottom with around 4"-5" of wood chip, covered it with cardboard, then topped the beds with garden soil mix. The chip decomposes over the growing season and the beds will need to be topped of with more mix. Your mileage may vary.
