Basil. There's no real order to auditioning or writing music. Each individual will approach things differently and will usually request a particular instrument, voice, lyrics or all. You can write a tune from vocals only or from a bassline or from a drumbeat or whatever. Doesn't matter. That being said I personaly have a fairly consistent way of writing.
First I like to create a good bedtrack which includes a steady drumbeat/loop, rhythm guitars and bass. That's the meat of the song.
Next would be to find lyrics and vocals. I like to get that in before keys or lead guitar which I see as the 'filler'. It helps to have vocals in so the lead and keys have a vocal melody to follow. They can play around the vocals in harmony with the lead voice as well as 'filling' the space between. I may throw a temporary lead in the song to show 'do not sing here, this will be the solo spot'.
Once the vocals are in then it's a decision of what extra instruments do you want to add. More vocals (backings and harmonies). Lead guitar. Piano & Organ. Other instruments such a Cello etc.
At any point I may put up a drumless and a bassless to replace my original bedtrack. Drum & bass 'generally' follow the drive of the music and not so much the vocals.
Vocals to me are the lead instrument. The bass, drums, guitars and keys shouldn't walk over the vocalist.
So basically 3 major steps.
A good rhythm bedtrack
Lead vocals
Then the filler. Backing vocals, more guitars, keys
Again you can really start with any instrument or voice. I've written lots of songs from piano first. But usually vocals are the middle step. I need those in to see what else the song needs.
Just my general rule of thumb.
Dave