There are a number of ways in which underground wiring can be performed. This article addresses the direct burial of a cable.
Only certain types of cable are suitable for direct burial without additional protection. Commonly used types are Steel Wire Armoured (SWA) cable, and other PVC clad cables that have a concentric conductor design where an earthed metallic sheath encloses the live conductions.
Other cable types would need to be protected in suitable underground ducting or trunking.
An underground cable will normally be laid in a trench, and then the trench back filled to cover it.
The cable needs to be buried deep enough that it is unlikely to be disturbed accidentally in future. There are no hard and fast rules here since circumstances will dictate and appropriate depth. In areas unlikely to be dug over or disturbed such as under a path or a lawn, then shallower burial is acceptable - possibly a spade's blade depth or around 12". For a cable passing under a flower bed, where one could expect the area to be dug over, A typical minimum depth would increase to at least two spade's depth.