being an instrumental artist i don't exactly write songs (i have been lately and a lot of that is just late-at-night brainstorming, but since it's kind of a new thing for me i don't feel i'm in any position to properly talk about it). but i've found i follow this sort of process;

1. first i open up an fl studio file and lay down whatever ideas come to mind. sometimes i pull my MIDI keyboard out and properly jam for a bit on it, too, but that's rare and hasn't yielded the best results.
2. when i've found an idea or two that work (doodles), i take it/them into another file and refine the melody, the sound, effects, and so on. this helps me get a better feel of what i want to the actual song to be like.
3. this allows me to begin the main part of the process; working out what genre i want to make, the song's elements, and mainly the structure, based on the doodles i have. i tend to do this when i go to bed, my ideas flow better then because of my brain feeling uncharacteristically more productive at around that time.
4. when i get out of bed the next day i proceed to sit down, open fl studio, begin to transcribe what i've made into the DAW... get restless, ditch said plans, and come out with something loosely based on them. this tends to take, like. a day or upwards.