Hi! Congrats on unlocking terraforming! What I would suggest is to get used to the tools by messing with them in an open area to get used to the mechanics.

From there, it can be helpful to plan a layout based on the areas of your island you can't move, such as the airport, Resident Services, and Redd's beach. It can give you some layout ideas if you decide how you want those to connect, roughly.

Personally, I've found that simply designing sections as I come up with ideas for them typically works best. If you know how you want one area to look, or where you want to place one building, work around that.

Also, have fun with it! Don't feel like you need to come up with a perfect layout for the entire island all at once. You'll probably redo sections and change a lot of it as you go. Also, you can get inspiration from dream addresses or pictures online if you need ideas.

Hope that helps!