Question 1: Why make an herb spiral? - Because you can put more herbs in a smaller space. Because some herbs like drier, warmer soil (top of the spiral) and some herbs like wetter, cooler soil (bottom). The plants that can tolerate the most shade should go on the North side, those that like more sun can go on the South side. This gives you a lot of options/ environments in one spot. Also, it looks pretty and will impress people who like permaculture.
Question 2: How? - Pile up about 1/2 yard of dirt in a sunny spot.
Start at the bottom, make a flat part in the hill, and then spiral around until you get to the top. It should look like a drill bit or corkscrew (depending on your prefer for wine or tools). Then you take bricks or rocks and stack them up a couple of layers high along the flat part. (We used bricks because someone gave them to us for free.) Now the bricks should be higher than the dirt behind them. Then you set in your herbs according to their preferences for light and water. Hot tip: Most culinary herbs prefer really good drainage and sun; but parsley, chives and chamomile can take more shade. After you put in your plants, fill in with a bit of extra dirt to fill it up. Water them in and you have done it. Enjoy fresh herbs in your cooking.