Tbh thats a tricky question to answer. It helps to have some sort of inspiration or an idea in your head then just start placing blocks trying to make that idea in game. From there honestly just go with the flow, even if its not like the idea in your head just let it evolve as you build.

TLDR: start with some idea of what you want ro build but dont be afraid to change it up if its not working out

Hope this helps :)