its hard to explain do a search on here as i did a diagram not long since,make as long as you possibly can,wrap all joints in plastic,surround with stone for drainage if you can,cement joints in bed if you can,make beds same size as a 3x2 slab as use this as floor,sides i block and 1 brick high,3x2 slab roof covered in plastic with a foot of earth,plastic dous work but half fill with soil to stop echo.make sure enterances are below ground level and facing away from prevailing wind.if make of plastic use pot for the ends