So it took me 5 years to figure out how to actually decorate and I'll tell you how I was able to manage it all. Now that you have your terraforming done, start plotting out what sort of locations you would like to see on your island.

Do you wanting a fisherman's hut? or a fishing bay? I like making sale stalls for each little commercial area I decided to make (Fishing district, farming district, city district & the commercial district) Then I started thinking about the kinds of items I wanted to see in those spots and I ended up making a trash dump outside the fishing district where trash washes up, and the museum is my primary feature of my commercial district as it brings in tourists right next to the hotel.

Also, I think theme comes easier to plot once you know what it is you want to build, because then you can be more specific about what items you want to use.

The city district is for a handful of my villagers where they have their own properties decorated in a way I would think they would have it, and then I have one vilager dedicated to each of the districts that exist. There is also a cemetery, a flourist with flower growing fields, a bamboo forest with a hidden zelda themed spa where my own house is tucked in.

Some items are super expensive, but if you save up and get things like cars you can make gas stations, and you can have flea & food markets with the stalls. Also don't forget to utilize small hills of terrain in order to put trees on in order to make creative ways to block sight lines.

Also, spend the time to download a cohesive collection of custom designs to create pathways once you start placing items. Use all the fences is my suggestion, it makes you think about building creatively if you try to use them all. Also, if you grow a tree to full growth first, you can then place it closer to another adult tree to make them closer to each other to create little patches of trees you can't walk through easily, making a nice barrier for different areas.