I highly recommend checking out Airport Design Editor. It allows you to make a package out of an existing airport which makes things a lot easier as you can start with what’s already there, and delete what you don’t want, rather than have to rebuild everything.
The latest version will be at the top of the forum. You can edit the airport in either ADE itself, or, do what I do, create the package of the airport I want to edit, then edit in the default Scenery Editor in the sim.