For me, saying the mantra until falling asleep means the following: I start saying my mantra when I notice that I'm tired enough to fall asleep. When the time is right, I tell myself "I'm dreaming" over and over in my head (while visualizing myself getting lucid and truly believing in it) and when my thoughts go elsewhere, I focus on it again. After about 10-15 minutes I usually just pass out (while laying in my regular sleeping position). The key is to not worry about falling asleep while saying your mantra. Let it just come naturally
If you would fall asleep while repeating it consciously it would be basically a WILD.