What is it called when you write something how it is said?

This is called "phonetic spelling", and most Words in English are impossible to nail down because of the various languages that contributed Words to English.

Still, most letters have only a few sounds and many spellings are obvious to those moderately fluent in the spoken language. The letter combination "sh", for example, has a consistent sound. However, that sound can also some from the spelling "cio" (spacious), "ti" (addition), and even "xio" (anxious).