Source: Ruth HartnupGeneral:
Source: Ruth Hartnup
Basically you sow the seeds to a depth equal to the thickness of the seeds.
If the seed is 0.5 cm thick, then you add a layer of topsoil of 0.5 cm or you dig a hole at that depth, put the seed in it and push the topsoil back over the seed.
For very small seeds this is of course impossible. You sow them on the surface and then press firmly on the surface of the soil. This will push seeds slightly into the ground.
Some seeds need light to germinate or light accelerates germination. In this case, do not bury the seeds or only partially.
Whether a plant needs light to germinate or not is usually mentioned in the documentation concerning the propagation of the plant.