Yoga nidra means sleep with a trace of awareness. It can relax the whole system, awaken the psychic body, give one experiences of the astral plane. When you practice yoga nidra you are opening the deeper phases of your mind. At this moment, your subconscious and unconscious levels of the mind open, meanwhile the physical body is relaxing.
Yoga nidra is a very simple practice. Before starting, choose a quiet room, wear comfortable clothes, lie down in savasana, close eyes and listen to the instruction . During the process, try not to falling asleep in order to gain more benefits from the practice.
A proper Yoga nidra usually takes around 40 mins, and before it start, you may make a resolve (sankalpa). The instruction is divided into few parts including the guide of relaxation, visualization and waking-up.
Here, for the first trial, you can use the below recorded instruction (without visualization).
Mini Yoga nidra: