I started by getting rid of all my rivers, fruit trees and flowers, then terraforming the land until I had all the layers of land roughly planned out! (This took me around 10 hours.

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After that I began adding little by little each day; Working on rivers, waterfalls and ponds, relocating trees to mostly completed locations and revising my rough cliff work by adding more detail! Every day I've also been relocating a service or villager home to it's permanent resting place, as well as constructing either a bridge or incline if I've planned/semi-completed the spot for it in advance! Once every building is in it's place, the final process will be decorating with flowers, furniture, weeds, custom design pathing, etc!
I would say though, that tearing down everything from the start can be quite daunting, on the first day or two I really felt like I'd ruined my town and regretted my decision - but I stuck with it for the long term and I'm so happy I did, everything's finally coming together and looking more and more like the island I had in my head! (The
Happy Island Designer github resource was also a godsend when it came to giving me a rough plan to go off in the beginning!)