I have a small notebook with ideas to decorate my island and villagers I'd like to bring to my island. Whenever I see something I'd like to replicate on my island I write it down, with a rough estimation of how much space it would take. For the villagers, it's so I don't have to always look up villagers on hunts, I usually have a small list if villagers I'm looking for and I already know their personnality, hobby and house design 
