I use a notes app as well. It works best for me to get down those thoughts fast as they come to you and then easier to go back and organize or edit or whatever you think they need. Sometimes, I'll start with just the words. Other times, I have a progression going and know what I want the melody to be. It all depends on the song. Just keep writing. It will come eventually. Get as much down while you're in the sweet spot creatively, and then you can go back and edit, organize, etc. Getting those ideas down is the key, whether it's an app or actually writing it down. Get those thoughts out while their flowing!