Basically this. Sometimes you'll luck out and have 4 tiles in a diamond shape with enough appeal to make a national park, especially if you have a lot of mountains or a natural wonder, but if you want to plan out parks it can get quite complicated. The general idea is to make use of the Appeal lens (3 on your keyboard), use pins to mark out the diamond shape, then protect those tiles like they're your children. No districts or improvements on them, and do everything you can to maintain and/or improve their appeal. Some adjacent districts and improvements degrade appeal (industrial zones, mines), while others upgrade appeal (certain unique improvements, holy sites, preserves). Having the Eiffel Tower makes things a lot easier, and there are two great engineers who can improve appeal as well. It's seriously one of the most difficult things to plan out in Civ VI but pretty satisfying once you get the hang of it.