In Notepad++, how do I split long lines in several rows?

Go to View -> Word Wrap. Make sure that is checked.

If you do want hard breaks in lines of text, you can use this regexp find & replace all:

Find:      (?<=.{80})\s
Replace:   $0\n

Will look for a space after 80 characters and replace the whole match with itself and a new line (replace with $0\r\n for a Dos\Windows file)

Reduce the window size of Notepad++ to your prefered size (move the right window margin). Then select all + Ctrl + I (Edit > Line Operations > Split Lines).

Done.