The diameter is the thickness of the seed. This means that a big seed like a bean would get planted deeper than a small seed like a tomato. Very small seeds like impatiens can be put on top of the soil and just pressed lightly into the soil by hand. Put 2 or 3 seeds in each container because not all seeds will start growing. The seeds will start better if they can be kept fairly warm, around 70 degrees Farenheit.
Many people like to cover the containers with plastic or a lid until the seeds germinate or sprout.
This makes sure they do not dry out as they first start to grow which will kill the young plant. Make sure the containers are not in direct sunlight if they are covered or the container can get so hot the plants will die. Take off any cover as soon as green leaf like growth can be seen. The containers can now be moved to a cooler spot. Give the seedlings plenty of light.