You need to get your island to 3 stars. Ask Isabelle what stars you currently have. Quickest way to get to 3 stars is to build fencing around houses, plant loads of flowers and decorate your island with outdoors stuff like camp stuff, things on the beach and gym and outdoor recreation equipment. Keep checking with Isabelle to see what star rating you’re on and once your hit 3 stars, you unlock K.K. The day after KK plays on your Island, Tom Nook will give you the Island designer app. You can then exchange your Nook miles for the various paths, cliffs and water permits which you use to terraform your island.