Yoga nidra, or yogic sleep as it is commonly known, is an immensely powerful meditation technique, and one of the easiest yoga practices to develop and maintain.
As you lie down, supported in savasana, all you do is to follow the voice that is guiding you. It is likely that you will remember certain parts of the meditation and not others. Every time you practice Yoga nidra you encounter a new experience—none of which is wrong. Falling asleep is okay too, as you will still receive benefits while the unconscious mind is absorbing the practice.