Let's suppose that your application is named MyGreatSoftware. We will put it under the /app/mygreatsoftware wiki path. The rule is putting all characters of the name in lower case, and using the minus character if needed.
So you have to enter the following URL into your browser:
http://gambasdoc.org/help/app/mygreatsoftware
The wiki tells you that the page is not available. Then you click on the Create link.
To fill the page contents, just open an already existing application page in editing mode, and use it as a template. Once done, just click on the Save button. That's all!