If you have gone to the Google add-ons page for Auto-Latex Equations already and added the app, then you can start by opening up a Google Doc or going to Google Drive > New document. Anywhere in the document, simply copy paste this: $$ 3^{4^5} + \frac{3}{4} $$. Now, from the top of the page, go to Add-ons > Auto-Latex Equations > Start, then click "Render Equations" in the sidebar that appears.

A full list of commands can be found at the AoPS Wiki. See this video walkthrough on Youtube to see it being used in action.