Hello, Ladies and Gentlemen. I have a newbie question and now sure how to do what I want to do (lol)

I have this idea in my head and I can picture the layout (well sort of) of purple, lavender and white blocks in different angles with lavender binding and a white back.

Knowing what I want is the easy part, putting it down on paper is another issue. How do you all go about executing an idea that you have into reality? Do you sketch it out yourself in a notebook? Do you have an app for that on your cell phone or is there a computer generated program that anyone knows of?

Thank you!!