How do you all tweak your cliffs and terraform them when designing?

Trial and error. U learn by building adding, removing, and changing them until u like what you have and change what u don't like. There is no magic pill for building or doing cliffs if that is what u are looking for.

Learn as you go, it takes patience and time, ugh!

@James you are correct, It does take time to create something you like and enjoy for your theme or just surrounding areas. @Lulu. Definitely, the hard part for me is deciding the location of everything, and layout

Search for online tutorials or associated videos of others try googling "acnh+terraforming"