The simplest way to build an outdoor dog pen is to purchase a ready-made pen from a pet supply, home improvement or farm supply store, and build it according to the manufacturer's instructions. Building an outdoor dog pen from scratch offers the advantage of customizing it according to the needs of the dog and the size of the space.
Stores such as The Home Depot, PetSmart and Tractor Supply Company sell ready-made outdoor dog pens that are easy to build using a few simple tools. These designs are typically constructed of framed chain link panels that are held together with brackets. Some pens come with fabric or chain link roof panels. They tend to be sturdy and easy to move.
An outdoor dog pen built from scratch is more difficult to construct, but can be built to any size. Chain link fence and welded wire fencing are both popular choices for outdoor dog pens. Choose the type of fence best suited to the project, and estimate the length of fencing and number of fence posts that will be required. Don't forget to account for a gate. Pea gravel is the best flooring material for a dog pen because it provides good drainage and a soft surface for the dog to walk on.
Select a site that provides shade for the dog's comfort. Outline the shape of the pen by putting a small stake at each corner and running a string around the perimeter of the stakes. Remove the grass from the area, and fill in with a few inches of pea gravel. Set the fence posts at the corners, and attach the fencing. Install the gate and a roof if required, and finish with a dog house or other shelter.