Mine is an iterative process, one that attempts to replicate the experience of writing and arranging with a band, all on my lonesome.
When you develop a song with a band, it usually starts with a more-or-less finished song that was written by one person on guitar or piano. It usually has a generic verse-verse-chorus kind of arrangement, but that's just the foundation from which each player contributes his own specialty to flesh it out.
It's an iterative process - you play the song as a band, over and over, and each musician gradually develops his part with each pass. The bass player won't know what he's going to do with it until he plays with the drummer, and the two of them play off one another. Keyboards and guitars develop a dialog, each creating space for the other, each modifying their part in response to what the other's doing.
This is a natural organic process that leads to interesting music. It's also the hardest thing to recreate when you're a one-man-band. You can't feed off others' creativity, can't try out complementary counterpoints, can't experiment as you play.
But you can do something similar. Start with a drum track that more or less follows the rhythm you imagined while you composed the song. Don't worry if it's dull and repetitive; you'll fix that later. Next, add a guitar or piano to lay out the chord progression and song arrangement. Don't worry if it's dull and repetitive; you'll fix that later. I usually add bass next; again, don't worry if it's not spectacular at this point.
I'll keep building up a song like that until I've got a bunch of dull, repetitive and disconnected tracks that lay out the arrangement. Then I'm ready to pretend to be multiple players, starting with drum fills and cues. Then think about how the bass should interact with those cues, then the other instruments until I come back around to the drums again. Now I've got some changes in the rhythm guitars and keyboards that a real drummer would react to, such as pausing when they pause or reinforcing their accents, advances and ritards. Bounce back and forth, letting each lay back or create holes for the other to fill.
It's not a quick process. An arrangement that might take a few hours' time with a real band could take days or weeks in solo. But at least there are rarely any arguments between the musicians.