Here's how I usually do it:
- I write a list of stuff I want to design, like cafe, library, flower garden etc. I try to think if there's anything inspiring me, like if there's one item I really like I can think of an area to use it and what other stuff will look good around it. I also like to make areas dedicated to specific villagers and what I think they would like.
- I also get inspiration from social media, sometimes I just straight up copy a design I like (just don't claim it as your own if you share it on social media, and credit the OG creator). I keep screenshots or I save it on instagram and Pinterest.
- I decide how I want to organise my villager's houses, like if I want them all spread out or if I want a few of them to live together in a neighborhood.
- I use Happy Island designer (or a physical notebook) to roughly decide where I want my areas to be, and to plan roads, bridges and inclines.
Aaand, that's usually it. I'm also not good at designing stuff and I never "finished" an island, but I did build cute stuff from time to time. Just remember to no stress too much and not to overcommit to your plans, they'll probably change and you might rebuild certain areas again and again. Don't try to do everything at one and get burnt out.
For the actuel "designing" part I don't have a lot of advice, use colors that you think look good together and experiment with custom codes? Like I said I usually end up copying what someone else did and modifying it to fit my island and my taste.
Good luck!