Really? "Just start."
Somewhere. Anywhere. Put a lyric in your head and just start plinking-down a melody to it. Keep the Record
button firmly mashed-down and record another "plinking" and then another and another.
(Go ahead. Your computer will never run out of memory.)
"Pick out the best one ... for now." Cut-and-paste it into a new track. Develop it. See where it goes. "Not so much? Don't(!)
erase it! Mute it! Try again. And again and again." You've got unlimited studio-time, and no one's paying union wage.
Ready to try chords? "1 - 5 -4" is "three chords and the truth.™" Start there. Just block it out. Don't like that?
Mute it, copy it, try again. (Save periodically!)
At some point along the way, somehow, "something magic
begins to happen." Whatever you do, "don't erase it." Just keep adding alternatives. Keep doing that until you fall asleep. (Time Machine: "Back Up Now.")
Next morning, and now with fresh eyes, review it. Review it all
. Every "take," every muted track. Within all of this you will begin to perceive the pattern of a brand-new and original song.
As you continue "working it," just keep adding new things. Never
throw anything away. ("New project?" Sure. Go ahead. Just don't trash the old one(s).) "Takes"
can be your best friend as you try to work out the chord-progression (or, "whatever progression") to accompany a particular part. Feel perfectly-free to accumulate them, then keep all of them!
Yes, every single one.
"From all this source material," you can then begin to construct
"your song ... or several." Truth is, "there is no single answer." There's more than one way that any song idea could go.