Step one is to place your stakes where your dahlias will go. Stakes are necessary for taller variety dahlias to keep them from blowing over during wind storms. Five foot steel fence posts are great for this purpose, especially in heavy soils. Some people use rebar cut to four or five foot lengths and of course the old standby of most gardeners is cedar stakes.
Spacing of your stakes will depend on your individual needs. Larger varieties are generally placed 30 to 36 inches apart and smaller varieties will be fine at 24 inches. Most people plant double rows (two tubers or plants per stake. Some people, however, plant staggered rows with only one tuber or plant per stake. When planting staggered rows, plant your tubers offset four to six inches from a center line and space your stakes 18 to 24 inches apart. The nice thing about staggered rows is that your watering hose can be laid down the middle of the row. You can also place more tubers or plants per row with this method. Space your rows 4 to 5 feet apart so that you have plenty of room to get in around your plants to groom and tend. It also helps to promote better air circulation, which helps prevent powdery mildew.
Once your beds are ready, stakes are in, and all chance of frost is past, its time to get your hands dirty. For average soils, most people plant their tubers about six inches deep. Plant eight inches deep in light, sandy soils and four inches in heavy, clay soil. Dig a deep hole and back fill to appropriate depth. Lay tuber horizontally with the eye end toward the stake. The tuber eye should point up and be about four inches from the stake.