While the manual tells you how to do everything, here are some simple instructions to get you started.

Click on the pencil image to the right of the rectangle area. In the edit-window you can click anywhere and just start typing.
When you are done you can click on the “preview” or the “save” button at the bottom of the rectangular area. When you preview, remember you will still need to click “save”

These three special code-sequences should be enough to get you started:

1. An empty line will start a new paragraph.
2. A "\\ " sequence will start a new line. This is backslash-backslash-space.
3. A sequence like [[page-name|Description of page]] creates a link to a page internally named “page-name”. If that page doesn't exist, it will be created.

All the rest is only icing on the cake…