Planting depth depends on the type of plant but there are a few basics that can be generally applied:
Rhizomes like anubias and java fern tend to do best mounted on rock or driftwood to ensure the rhizome isn't buried but it is possible to plant them directly in the substrate so long as only the roots are buried and the rhizome is only laying on, not in, the substrate.
Plants with discreet root zones--most often those that form a rosette of leaves like swords, crypts, some vals, etc--should be planted so that the line of demarkation between the roots and green leaves or stem are right at or only *just* below the surface. If you bury it too deeply, the leaves will rot. If you're using planting tweezers, it helps to hold the tweezers vertically aligned so the tips are at the end of the roots and much of the root system as possible is held wthin the body of the tweezer so they are inserted vertically into the substrate. Sturdy specimens can be initially pushed deeply into the substrate then pulled back up to level--but you run the risk of injuring the plant and this can disorder layered substrates.
Stems that do not have discreet root zones and will generate roots at any node can be planted as deeply as needed to hold firm. Roots will grow along the buried portion though care must be taken not to injure the stem in the process. Often the stem is crushed or creased at the level of the substrate resulting in the root mass rotting and the stem ultimately floating free again.