Step one: Make your own potting soil:
One part perlite
One part sand
Two parts compost
Two parts peat moss
Note that this does not include fertilizer. This will only get your seeds and plants started - they need a more nutrient-rich soil once they get growing.
Step two: Fill the bottom third of a pot with peat moss.
Dahlias love soil with a lot of tilth - and they are shallowly rooted so adding peat moss in the bottom of the pots - and planting it with the tubers - will help your soil.
Step three: Add a big scoop of your potting soil.
I add enough potting soil to fill another third of the pot.
Step four. Add your tubers.
Make sure the little eyes or buds are facing up.
Step five: Cover the tuber with more potting soil.
Dahlias are not deeply planted so you really only need enough potting soil to cover the tuber entirely. I do not use peat moss to cover the tubers.
Step six: Do not water.
Water and rot are your dahlia's worst friend. Usually the compost in your potting soil has enough moisture to get the dahlias going. You can water the potted up dahlias - but do not over water. The soil should not be soggy. Once the dahlias sprout you can water them like other plants.
When the plants are about 18" tall you can pinch the tops to encourage better growth.
