The concept of yoga nidra is actually an ancient Indian tradition associated with Hinduism and Buddhism. In modern times, the practice taught by Satyananda includes eight stages (internalisation, sankalpa, rotation of consciousness, breath awareness, manifestation of opposites, creative visualization, sankalpa and externalisation). The journey to learning, understanding and practicing yoga nidra is a personal journey. Many yoga nidra sessions are taught professionally in a yoga studio with many years of guided practice.
“Yoga nidra can serve as a pathway to freedom without creating bodily stress. A samskara (mental groove) is formed by a repetitive thought or habit that is deepened in the mind and body, creating a mental impression (groove) over time. These impressions can cause negative reactions and emotions which prevent a unification of the five layers (koshas) of one's self. If these negative emotions continue to plague you, the mental/emotional layer of the self is unable to integrate with the other layers: physical, energetic, higher wisdom, and bliss body. This is where yoga nidra can be especially helpful.” 
The practice has been known to reduce anxiety and help individuals fall into a deeper sleep. By practicing a few times a week, you will likely notice yourself falling asleep quicker and falling asleep to the sound of your natural environment.
The best part is, you cannot practice yoga nidra incorrectly. It is a practice that can only be captured by the individual through a deep, restorative practice. Each time you fall into a deep sleep, you are able to remember elements of that sleep that felt relaxing. These states of mind are called savasana. By following savasana, you are able to listen to the voice within that helps guide you to sleep. It is accomplished usually by laying on your back and practicing the art of yoga nidra.
The science behind why yoga nidras work is fascinating. From the moment you fall asleep, your brain is beginning to deepen its connection with your body. From the alpha stage of sleep, you drift off into a deep alpha or high theta brain-wave sleep state. During that state, you begin to dream (REM sleep). In the theta stage, your thoughts actually slow down to 8 thoughts per second. In this stage, your brain is highly susceptible to learning. You can equate this to creative individuals who often have theta waves flowing through their brain. Even so, this stage can help one achieve the pit of nothingness.
“After theta, you are guided to delta, where your thoughts are only 1 to 3.9 thoughts per second. This is the most restorative state, in which your organs regenerate and the stress hormone cortisol is removed from your system.” 
After this state, you can welcome yoga nidra into your state of mind. The guided yoga nidra experience takes you down into an even deeper brain-wave state—one that can’t be reached through conventional sleep. In this state of mind, your brain enters an interesting state of thoughtlessness. You are awake but you surrender to your inner consciousness. It is challenging to reach this state and even those experienced in practicing yoga nidra can often find themselves tapping into this state on a rare occasion.
While practicing yoga nidra, you may notice you are asked to bring your attention to the center of your eyebrows. This brings attention to your third eye. Focusing on your third eye brings attention to your inward consciousness. It will help stabilize your emotions and bring your thoughts inward.
How to practice yoga nidra:
1. Connect with your inner voice
Listen to the voice in your head. What is is saying to you? Before falling into a deep meditation, you may find your mind racing. Speak to yourself using calming tones that seek to provide your mind with inner joy.
2. Welcome your emotions
When you are falling into a deep meditative state, do you notice yourself feeling a range of emotions? You are possibly experiencing a series of emotions you felt throughout the day (and not all of them are joyous). On particularly difficult days, yoga nidra comes into play because it can help separate your inner feelings with the physical body.
3. Experience your emotions
While in a deep meditative sleep, you will most likely encounter more feelings than the feeling of joy. Experience those emotions in the vast array they may come at you with.
4. Observe your senses
While attempting to achieve a high meditative state, notice your senses. Are they changing? Affirm your senses and connect with the sense that comes to you first. Take each sense slowly, absorbing the entirety of the feeling.
5. Transition to your waking state of mind
Slowly bring yourself back to your waking state of mind. Note your gratefulness to be in the present moment. Transition at your own pace. If you do not feel ready, transition at a slower pace.
Yoga nidras can be recorded live or conducted in person. Many cities record nidras in yoga studios, meadows and woodlands and expand to almost every common language such as English, German, Italian, Russian and Spanish. You can find the most widely practiced nidra by Richard Miller here.