Scatter the seed thinly over the surface. Good advice that's hard to follow, but do your best to spread it around. Sometimes you can find pelleted seed, where carrots or lettuce or such have been coated with material that makes them easier to handle and space correctly. Fine little seed such as begonia or primroses needs no further covering. In fact, little seeds often need light to germinate.
Larger seed is covered one to two times as deep as the seed diameter.
The best way to thoroughly saturate the soil for this all-important first watering is from below. Soak until the surface soil looks damp. Let drain, and put under grow lights or other well-lit place. The trick now is to keep the pot from drying out. I like to use a plastic cover that fits over the flat that holds the pots or cell packs.