Do you say write as simple possible or simply possible?

The most grammatically correct and natural way to say it is "as simply as possible".

Here's why:

* "As simply as possible" is a comparative phrase that emphasizes the desired level of simplicity.

* "Simply possible" is grammatically incorrect and doesn't convey the same meaning. It's unclear what "simply possible" actually means.

Therefore, "as simply as possible" is the best choice.