Yoga Nidra translates into "yogic sleep." It is an accessible and restorative yoga practice that follows a carefully crafted script that guides the practitioner towards a state which "demonstrates the symptoms of deep non-REM sleep, including delta brainwaves, while simultaneously remaining fully conscious." 
It can be a gateway to explore the great mystery of what and who we are as well as a wonderful tool to connect to Source. All you need to do is lie down and listen.
In other words, we put the body into a state of sleep while the mind remains aware. Then we move through the koshas , or subtle layers of our being, to allow for the opportunity to arrive at the state of Yoga Nidra.
The state of Yoga Nidra is also known as Turiya. Turiya may be experienced as a feeling of oneness. It is described as the "final liberation from ego consciousness and is united with infinite spirit...it pervades all levels of reality as superconsciousness." 
What is a Sankalpa and why should I use it?
Sankalpa can be used as a way to liberate limiting or conditioned beliefs we may have about ourselves. It is a short and sweet affirmation, always in present tense and positive language. Often recordings will suggest some to choose from, but if you have one you are already working with, feel free to use it. Some Sankalpa examples are: I am healing. I am valid, I am whole, or I am loved.
Feel into what it may be that you need the most, work with it for a while, and notice how it feels and evolves within you.
It is used twice in the practice. First, in the beginning, as a way to establish which one you are using. Then again towards the end of the practice to integrate it with your subconscious inside the state of Yoga Nidra.
We can work together to find a Sankalpa for you.
Do I need any special gear to practice?
No need to go out and buy any fancy props or gear. Some of us may want to use a blanket, an eye covering, and a pillow under the knees. These are all things you may already have at home.
Some may need extra support to find comfort, like carefully placed blocks or bolsters. They can be wonderful to use for those who are pregnant, or working with other physical discomfort that make lying on your back difficult.
What if I fall asleep?
It is said that you will get whatever it is you need from the practice. If you tend to fall asleep, it might mean that you need more sleep. If you want to try to stay aware in the practice, try setting up a dedicated space for Yoga Nidra that is not where you usually sleep. Lying on a mat on the floor instead of your bed may help in staying awake.
Is there a recommended time of day to practice?
You can practice at any time of day, some of us may only be able to at certain times and that is OK. Remain open and curious of trying different times to see how they may change how you experience the magic of yoga nidra. So, if you tend to practice in the evening, try switching it up and practice in the morning or afternoon.
When you have downloaded a recording you can experiment and practice whenever and as often as works best for you.