Then tap it gently on your work surface to settle the soil. That works better for me than firming the surface from above. You want the soil mix nice and open so fine roots can penetrate easily, and lower than the rim for ease of watering. Scatter the seed lightly over the soil surface. One common mistake is to sow too many seeds, perhaps merely because there are more in the packet.
Crowded seeds grow poorly and are more difficult to transplant later on.
If the seeds are very fine, like begonias, do not cover them with more soil mix. The larger the seed, the more covering it usually prefers. Cabbage and zinnias get a thin layer, nasturtiums and morning glories a thicker one. Watering is next. I prefer to water from below the first time.