Rather than try and get your head round all the text and howto use documents, please download a sample Excel Spreadheet and have a look at it.
The spreadsheet is self documenting and can be used by itself (if you fill in the fields) to upload and test the system. HMRC forbid cutting and pasting the data manually for the returns but there is no nothing to stop you manually testing the system - but you shouldn't make the return 😉.
You can download a LibreOffice template if you are using OpenOffice or LibreOffice.
There is also a sample (undocumented) Windows/DOS text file template to download if you don't want to go to the spreadsheet route. The text files can be created programmatically or from a macro in a spreadsheet.
And the Linux/Unix version can be downloaded here.