When you're done with the customization, you can start writing. Content Editor is WYSIWYG (What you see if what you get) and get basic text styling options.
You can either start writing in the editor or copy-paste your content from some other writing tool. Surfer will give you instant feedback on the optimization progress:
If you want to see where exactly each phrase is used in the text, you can click on the box with it and it will highlight on the text.