Look through the thread jaizei posted for good ideas.
I made my Russian pen walls 12" tall with a 4" perimeter lip. It's just painted plywood. The lip keeps them from escaping.
Plant it with lots of grasses, weeds and other edible plants. Add water sources and hides.
If you have predators/dogs in the area you can make a cover to keep your tortoise safe.
Here is mine. I wish I would have made it bigger and planted more plants in the ground. Now I have many updates I want to make when Spring rolls around. Wish I would've done it the first time!