These are questions I do not know how to answer. It can not be explained, in my opinion. The way I cannot explain how I feel after seeing the sunrise over the ocean. Often it takes weeks, months, years to finish a song. However, after I post this I will head to a writing "appointment" with another writer where we will probably start and finish a song in 4 hours. This is one way to write. But even there, even in the fast food of songwriting, my best moments are when I dive in, when I commit, when I give. I do think that the best songs are written exactly as Annie Dillard explains here in her book "The Writing Life". I have never heard it explained quite this way.
This, my friends, is how you write a song:
To find a honey tree, first to catch a bee. Catch a bee when its legs are heavy with pollen; then it is ready for home. It is simply enough to catch a bee on a flower: hold a cup or glass above the bee, and when it flies up, cap the cup with a piece of cardboard. Carry the bee to a nearby open spot-best an elevated one- release it, and watch where it goes. Keep your eyes on it as long as you can see it, and hie you to that las known place. Wait there until you see another bee; catch it, release it, and watch. Bee after bee will lead toward the honey tree, until you see the final bee enter the tree. Thoreau describes this process in his journals. So a book leads its writer.
You may wonder how you start, how you catch the first one? What do you use for bait?
You have no choice. One bad winter in the Arctic, and not too long ago, an Algonquin woman and her baby were left alone after everyone else in their winter camp had starved. Ernest Thompson Seton tells it. The woman walked from the camp where everyone had died, and found at a lake a cache. the cache contained one small fishhook. It was simple to rig a line, but she had no bait, and no hope of bait. The baby cried. She took a knife and cut a strip from her own thigh. She fished with the worm of her own flesh and caught a jackfish; she fed the child and herself. OF course she saved the fist gut for bait. She lived alone at the lake, on fish, until spring, when she walked out again and found people. Seton's informant had seen the scar on her thigh.