Use the "full screen" button to increase the size of the window and you can edit better.

In the upper left, choose the format of the text by selecting from the dropdown menu:

- Paragraph: for normal text

- Chapter of the first level, second or third: to highlight the titles of the sections and build the index. Those statements that highlight in this format will automatically appear in the document index (table of contents).

Use the options Ctrl V or CMD V to paste and CtrlC / CMD C to copy, Ctrl X / CMD X to cut (in some browsers buttons doesn´t work)

Once you finish editing do not forget to save your changes by clicking the bottom left button Save or Cancel if you don´t want to save the changes.

More information here