A song is already a song with just a single instrument and a melody. I like to start with a piano and a melody that I humm into reason so I have this simple piano and vocal or flute template. It's a simple template with vocal, piano, flute and a lead synth that I can either play with keys or the ewi.
I like to create this barebone song, and probably get a strong piano riff and melody. I like to get the main aspects of a song's form still in this part of the development because I feel I can evaluate better the artistic and creative aspects like if the melody is interesting, the chord progression and so on. And hopefully get the two main parts, A and B, verse and chorus. All this is still just with piano and vocal or a lead like flute or ewi with a synth.
If the song has lyrics and they are mine I might do everything at the same time and get it more organically just thinking on the subject and building melody and lyrics at the same time. If its a lyric I did before I pick it up and try to define a melody and harmony for it.
In the rawest form I have to be happy with melody, rhythm, base harmony and and also If it has it, lyrics. But the song must have a verse and a chorus and tell a melodic and harmonical story.
I like to think a song it's like a woman, you only see her real beauty without makeup. But if you like her without makeup... (change gender accordingly...

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I also like to work with a song form in mind, it's not a complex form, I brang it from the type of arrangers I used when I played at dances when I was younger and most of the songs from the 70'to the 2000's have it, whereas lots of time nowadays we've lost some form diversity (Sting complained on an interview most current pop songs don't have a bridge, I still like to have a bridge and he also explains why and how a bridge can spice the song and wake up the listener to get it's attemption). A bridge can be a lot of different things, from a third counteracting theme on a lyric that also benefits from a different melody and harmony, to a rap section, to a melodic interjection with a different instrument. So I start to try and have this form in mind: intro, verse, chorus, verse2, bridge, ending. Depending in the size these can be rearranged into for example intro, verse, chorus, verse 2, chorus, bridge, chorus, ending. Or use the bridge as a prechorus or pre verse and bring it sooner. But the parts are more or less there. Variations like transpositions or breaks can get done later but the main form of the song (a, b or verse and chorus need to be done in the raw).
From the raw part I get into the instrumentation and arrangement (a lot of the things I've been doing start at this part because I've been doing lots of arrangements for other artists), review harmony if it is too simple, decide the intro and ending and try to come up with a bridge. For the bridge, if there's a lyric I'll try to come up with a melody for it, if there isn't, I try to review the arrangement with some techniques like transposing, reverse harmony, reverse melody, modal changes (like going from major to minor or to the relative minor or morph things. When inspiration lacks, musical theory can be a good tool, reason players, like the chords and scales or the big chord player can be helpful too).
The same applies to intro and endings, an intro can even just be a simple chord progression from the harmonic main instrument or be something more complex with different melodies and what is usually called a hook, and you might have a first presentation for that hook. YOU have to adapt to the style and check complexity, pop benefits a lot from repetition, other genres benefit from diversity and improvisation.
The rawest part is simpler but IMHO harder to get. It depends on inspiration, drive, sometimes mood, and I like to think it's still driven by human artistry, feelings, novelty, and musicianship, and the times I got more fun with music is when I do this with other people, friends and other musicians. It makes the process more organic, and when you understand the rest is more or less make-up you get to finish more songs.
OK, so if the song has vocals I also do raw vocals adding vocal harmonies with pitch editor and when The arrangement is finalised I re record anything that is audio. If it's a band they all come in at turns (I like to work with the bassist and drummer in te same session) and record over the midi mockup or have a guide track. If it's a hibrid (midi + instruments + vocals) it's the same but with less instruments but either way vocals are usually the last part to get in. After this comes the cleaning and editing part, where I work out audio errors, vocal tuning, a final passage over midi stuff like wrong notes or bad timming or really odd velocities that affect later dynamics on the mixing. I also take care of project tidinyess, like track and channel ordering, coloring and base groups in the mixer (I like to have 8 groups each for source type).
At this stage I can start mixing and then go to mastering, but I guess that should be matter for other thread.
I hope it doesn't seem too industrial or formal but this is the way I Learned with my masters and I've taken some aspects of this and that person along the way. And the idea that you can judge a song in its rawest form is a great way to really get musical ideas flowing without getting lost on other aspects.
Cheers,
MC