- Press <Esc> to enter command mode
- Press <Ctrl> + <V> to enter visual block mode
- Move <Up> / <Down> to select the columns of text in the lines you want to insert text
- Hold down <Shift> + <i> and type the text you want to insert
- Press <Esc> , then wait 1 second and the inserted text will appear on every line you selected in step 3
- Press <w> (To save) And <q> (To exit Vi), if you're done editing this file
Credit: Original Tip found at https://stackoverflow.com/a/253391