<< I can visualise things in my head as 3D, terraced gardens, ponds and the like, but transferring them to CAD is just alien to me

You are not obliged!
Just draw or paint them on paper with pencils or brushes!

By Calatrava (a cool guy ! )
http://darani-blog.blogspot.fr/2011/09/santiago-calatrava.html
first drawings

in reality


Preparatory sketch for Wave, 2001. Watercolor and graphite on paper.