Find a spot with lots of sun. Dahlias need 6-8 hours of sun each day to flower regularly.
Make sure the area is well draining, if the ground is too wet or clay your tubers might rot. mounding your dahlias by surface planting on self made hills will help
Work in a good layer of compost and a small amount of sheep’s pellets or blood and bone.
Before planting, place a stake in the spot you intend to plant. Place it prior to planting as tubers can be easily damaged.
Tubers don't need to be planted deeply. 10cm of soil on top of your tuber is about right and plant the tubers at roughly 30cm apart. I plant mine closer for a hedging effect but this requires a large number of dahlias and digging up to divide yearly.
In the hole you have dug, lie the tuber on its side with the shoot pointing up. Don't water until it has visible foliage above ground and only if it hasn’t rained for a week.
Once your dahlias have foliage, and are roughly 12 inches high you need to pinch. To do this use sharp scissors ( not the square type ) and cut the top central stem down to just above the next set of leaves. This will encourage more stems and more flowers and create a stronger bushier plant that will be less lanky and will hold up to wind better.
Once your dahlias are starting to bud, maintain a regular water schedule ( weekly not daily ) and pick off and side buds that form ( see the notes on “pinching and cutting back for longer stems” to read more info on why and how. )
I usually add some dahlia fertiliser from bulbsdirect.co.Nz once flowers are beginning and again in mid summer. Remember to go low nitrogen, as dahlias will give all foliage and no blooms if you give them too much. Tomato fert is a good cheap option if you are stuck.
Dead head regularly to encourage flowers as the more you cut the more it blooms.