As far as the soil is concerned, make sure that it is well-drained. You can also replenish the soil by using compost or manure. In case the soil is a bit lighter, adding manure will facilitate the absorption of water and nutrients.
The seeds should be spread on the top of the container. It would be best to use a moist potting mix. Since the seeds are very tiny and difficult to manage, planting the seedlings would be more convenient.
Watering and Fertilizing
If you have planted this plant in pots or window boxes, make sure that you water it frequently. The new plants need to be watered more often, but make sure you don't go on a watering spree. You must also add suitable fertilizers as that will facilitate profuse flowering. You can fertilize using a liquid fertilizer that is meant for flowering plants.
Don't forget to prune this plant. You must remove the spent flowers and weeds. If you don't remove the weeds, they will compete with petunias for nutrients, which might hamper the growth of this flowering plant. Pruning and pinching is also important, as it facilitates profuse flowering and discourages excessive vegetative growth.