- Press Esc to enter ‘command mode’
- Use Ctrl+V to enter visual block mode
- Move Up/Downto select the columns of text in the lines you want to
comment. - Then hit Shift+i and type the text you want to insert.
- Then hit Esc, wait 1 second and the inserted text will appear on every line.
For further information and reading, check out “Inserting text in multiple lines” in the Vim Tips Wiki.