I could not find any pictures on here. I know they are here, I just can't find them. Would sure make it easier to understand.
Build a frame to hold the buckets horizontal. Either put uprights on the sides of each or put them close enough together so they cannot role. You can cut the lid in half and use it to help hold in the nesting material, but it is probably better to use a board across the front to hold the nesting material in. This will stabilize the bucket and give them a place to land when they are getting in or out of the nest.