Well...first I usually start decorating my house. When it came to the outside it depended largely where I felt it was needed. Usually I try to make the area around my house well decoratee but its not typically where I start decorating.
I know I began at the resident service area as I wanted to build out the town/neighborhood. Then I began around my home and worked my way from the back to the front. I was also a bit all over the place because I wanted to mark down certain terraforming landmarks.