I'd say, treat it like a lump of clay. Got a couple story arcs we could go in? cool, toss it in. Know 'kinda where you want to go with some personality types? awesome. push it all together. Now, which do you feel like working on today and build that up a little. A painter would then think, which parts need the most attention? work on that a little.
I think there is not a hard-fast check list to follow or an ordering that is important here, gradual build up comes from different places. Here is an example of what came to me today. The following are two of my characters that I've been refining from some time. The winged creature is also my art and was built for something else as a larger beast. Turn that around and make it a gecko++ sized shoulder pet for the female with large enough wings to drape over her shoulders, that would be a pretty cool combination. And now her personality has been altered to something much more dramatic. so, as you work it's also cool when those happy accidents or turn of events put it all together. No need to set everything in stone right from the get go.
Start with a clump and polish it into something shiny.