How to build a ladder over a fence?

The property my dh and I live on backs up to a large nature park which has woven fence with two strands of barbed wire at the top that goes around the boundaries. The fence is only about 4 feet high. We want to be able to go from our property to the nature park. The reason for the fence is because the nature park is surrounded by farms and they don't want cows, sheep, etc...getting into the nature park. Does anyone know where I can obtain instructions to build a ladder or ladder stile that goes from one side of the fence to the other with a small platform on top. Thanks for your help. Deb