Have one massive area dedicated to flowers, set up paths, and place furniture and other items basically everywhere you can, definitely plant trees too, it'll help your score
Another thing, follow every direction Isabelle gives you, there's no such thing as overkill
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0 ups, 2y
yea
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0 ups, 2y
btw you don't need to place your items, flowers and other stuff perfectly decorated. you can just stick them anywhere and you will up your islands rating