1. Add a string (comment for example) at the beginning of several lines:
    • Use Ctrl+V to select the first column of text in the lines you want to comment. (using up/down arrows to highlight the portion needed)
    • 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.
  2. Add a string at the end: 
    :%s/$/string you want to add at the end/g