It can be hard, since the idea is to basically fall asleep while remaining awake. So obviously the two most common outcomes is you either stay awake and don't enter the dream, or you fall asleep and fail to be lucid. As others mentioned, it might help if you try it earlier in the day closer to when you have woken up in the morning. It might even help if you wake up a little early and then go back to sleep and try it.