For a simple mulched dog run, fill in the space you dug out with mulch, evening it and leveling it out with a level-headed rake until the mulch has completely covered the space about 2" above ground level. Frame the mulched area with railroad ties or similar large pieces of lumber to keep as much mulch as possible inside the area. Think of it looking sort of like a sandbox. Mulch or similar material such as wood shavings works well because it is easy to clean up and to replenish when needed. Using a rake, dog waste can also be mixed in and covered up with the mulch and it becomes almost composted. Over time, though, all the mulch will need to be replaced even if you’re diligent in cleaning up soiled spots regularly to avoid odors and other potential issues.
Cedar mulch can often be a good choice because the oils present in it can repel insects. Just watch your dog to make sure he isn’t ingesting cedar mulch, which can lead to health issues.
Train your dog not to eat any kind of mulch if you notice that it is happening.
Soak the mulched area with water from a garden hose regularly to keep urine odor from becoming too strong.
If you’re building a dog run with a concrete floor, spray the dug-out area with water from a garden hose so that it is soaked. This will keep your concrete from drying out too quickly when you’re pouring. Fill a wheelbarrow with concrete mix and water as directed by the manufacturer’s instructions located on the package. Keep in mind that it may take several wheelbarrows full, depending on the size of your run. Pour the concrete from the wheelbarrow into the dug-out area, spreading it out into corners and edges with a shovel. Fill the area this way until the concrete is just above the sides of the form. Even it out and level it using a board as a “float”, running it across the top of the slab until it’s even with the sides of the form and flat. When the surface has been smoothed, let it set for the amount of time recommended by the manufacturer.
If you feel that pouring the slab will be too much for you to handle on your own, consider having a professional come out to pour and smooth out the concrete.
Be sure to wear safety glasses, gloves and protective clothing while working with concrete, because it may irritate your eyes and skin.