I have, such a dang long extensive process. It works for me thou cause my mind loves making lists.
1 Choose a theme that you are dead set on
2 Choose a colour scheme (if you have one) and choose what flora you'd like to use the most (flowers, shrubs, trees, ect...)
3 Make an album (I make mine on my phone) of all the inspirational pics of said theme, weather they be of other players builds, stuff from other videogames or even irl inspiration. I make 2 albums; one for all my exterior town stuff and one for all my interior home stuff.
4 Take notes (I literally write this down in a dedicated AC notebook but you can also use a memo app) of reoccurring things you see in those pics and that you want to incorporate. For example, my main island is a rustic nautical town. I saw alot of dark coloured buildings and old worn paths with overgrown grass that I wanted to incorporate into my island.
5 Take note of what major and minor features you'd like to have. For example, I wanted my main to have a grand entry way, a decorated shopping plaza. For my side island I wanted a marsh land, a corn or grass field, and a big fancy land made fountain of worship. Write these down so in the next steps of the process, you don't forget them. Write down every idea you want, even if you think you won't have enough space to incorporate it.
6 Choose your animal villagers (Optional)
If your town has a theme and you want the villagers to match, look them up. I go by exterior of the house and sometimes what vibe the animal themselves gives off. AnimalCrossingWorld has an excellent article showing all current villager exteriors including Sanrio (thou I think it's missing Vivian's house). We obv don't know the exterior for the new animals thou, so none of them yet. Choose the 10 you'd like to have, or you can have a few backups incase some are too hard to get.
7 Look up all possible designs you'd use for your island. Look up several path options, and write down somewhere too if you want to use the default paths somewhere. Also include all accent patterns, stuff for stalls, maybe a grwas accent design, designs for the interior of your home, ect... I even go as far as looking up the clothing patterns I want.
8 The hard part; Make a rough map of your town. There is software to do this online, but I don't like using it. What I do is take a grid paper and make a rough design of my town map, marking down around what acre things that can't be moved are in, such as the airport, Resident Services, Redd's Beach, the river mouths and even the large rocks on the beach.
9 Next to that map, write down all the things you are GOING to need. Shops, Museum, Campsite, 10 villagers houses, your chars house and any alt players houses you have. Keep it by your map so you don't forget to make space on your map for them.
10 The hard part, part 2, make your general layout. This is the hardest part for me. I constantly re do my layouts. This part is really up to you, decide where you want everythin. I make rough areas of whare I want to put stuff. Cliffs and their levels, bridges and inclines, do you want awake or a river there? Where are you going to place all the things you need and all the things you want. For me, I make sure everythin has a clear path leading to it, unless it's supposed to be hidden. Take into account space you'll use if you want somethin like a decorated plaza or villager yards, which will take up more space.
10B (Optional) Maybe move all your buildingson the beach and flatten your island if you feel it helps. Depending on how far you are into developing, you may not need too.
11 Halfway there, just do it. This is the part where you do all of that planning you did, into the game now. For me I go acre by acre, starting with my airport entrance,since that for me dictates where my path begins. Here I start making out all the spaces where I'm going to put stuff like plazas and buildings. I also build the outer edge of cliffs that I will eventually need to fill in. I do things just to get an idea of how it may look. Same with rivers too. This way of you make a giant cliff, then decide you hate it, it won't be too much to break back down again.
12 Really alot of trial and error. There may be changes from your original plan. Maybe now you see you have less space than you thought, or possibly more.
13 Once you have an idea of where you'd like stuff, you can start moving your buildings and putting your paths down. I put my paths down first since that dictates for me where things go.
14 Once your buildings and plazas are in place, decorate with furniture and gardening.
15 Fine tweak stuff, such as stunting trees, adding weeds, flowers and shrubs, experimenting with furniture, add fencing, ect...
And what's very important to me, KEEP REFERRING BACK TO YOUR NOTES. I find that if I don't keep looking at them, that I tend to deviate from them and end up unhappy with my final result.
And some steps can be switched around too, this is just how I do things. Like I said, very extensive, very long. But making phyical notes and lists help keep me focused. Hopefully this will help you some, an happy decorating!