For some of the tutorials it is required that you use a plain text editor, for example vim, Notepad, emacs, Kate or gedit.
There is no tutorial on how to use one, as the editor used your choice and what editors can be used changes based on your computer setup.
A plain text editor stores only ‘plain’ text in the files it creates, with no formatting information (e.g. colours, font sizes or what font to use) stored with or in the file.
It is this that is important. Other programs that are not plain text editors such as LibreOffice Writer and Microsoft Word will add formatting information to the file, rendering it incomprehensible to a program that expects a plain text file.
Notes (Windows specific):
If you use Notepad it will implicitly add the .txt extension after the filename that you specify in the save dialogue box. This is unwanted and causes problems. To avoid this enclose the filename in speech marks (” “) when naming the file.
When reopening a file and saving it again the file extension won’t be changed. So you only need to do the above once per file.