It is funny that this should come up. I too, just bought Dahlia bulbs. If you look at them closely, the new green shoots come from the top 25% of the tuber or bulb. That is the end you should plant near the surface. The other end of the tuber should possess the vast majority of the roots.
From what I read recently, I am using containers but they could be planted in beds on the ground. I am using Miracle Grow potting soil. The planting time should be after the soil temperature has reached 60 degrees.
They like, prefer, morning sun, well watered but never soggy or wet. Do NOT fertilize right away. Wait a couple of weeks. They should bloom by mid summer right up to first frost!