Before you start working with your documents, create local copy by download them to your device.

• Call context menu by long pressing on selected document in list
• Select "Download" option
• Downloading document allows you to edit document

Or create new one if you do not have any.

• Press "Menu" button to call main menu
• Select "Add document" option
• When you finish with you document call "Save" option or/and return to document list

When you finished with document editing, use "Sync" option from main menu, to synchronize changes in ALL local documents with Google Documents.

Or use "Upload" function, from document context menu, to sync changes of particular document back with Google Documents.