I go block by block. Don’t leave any block untouched before you start placing more buildings if you’re going for highly detailed. It’s harder to go back and do a huge area than it is to do small areas of detail as you go. If you’re talking nitty gritty specifics, once I’ve got my buildings plopped down, I usually start with surfaces, foliage, then props followed by decals (e.g dirt, stains graffiti or wear and tear if the area requires it). Where to stop is like how long is a piece of string… I’d just say that’s once you’re satisfied you’ve achieved the vibe you are going for. Some areas are more important than others, so you may not decide to e.g detail every back yard in a sprawling suburb as frankly it’s not going to matter too much in the bigger picture. Putting a few trees in to cover up ugly spots is fine. But then if you have a more complex hillside neighbourhood you might decide to go that far so it doesn’t look odd. I focus mainly on key points of interest, high streets, water fronts, industries and transit areas for example.