Put your dog on a stay at some distance from the obstacle. When you first start, make the angle very small, so your dog won't always choose the obviously shorter path. Call the dog. Once your dog has the idea that the commands mean something, start adding hand signals. On the course, you may be gasping for air just when your dog needs direction.
Dogs follow body language much more than they follow verbal commands.
Vary between using voice, using voice and hand, and using hand alone. As your dog develops understanding, you can set up several obstructions, and try to guide your dog around them solely with commands. This is hard . Start with straight lines. Then add curves.