I write about a page about what the application is, why it exists, who will use it and what it will do. After this I create headers for each major part of the application, which each contain bulletpoints for features and such. I then decide on how the database tables will be formatted and again use bullet points for displaying each field, for example:
- Field Name
- --Type: Varchar
- --Length: 255
and so on.
I also look at existing comparable solutions and note down their features which could be included in my own application.