This replaces the beginning of each selected line (use visual mode to select) with "//":
:'<,'>s!^!//!
From: https://stackoverflow.com/questions/253380/how-do-i-insert-text-at-beginning-of-a-multi-line-selection-in-vi-vim
" This replaces the beginning of each selected line (use visual mode to select) with "//": :'<,'>s!^!//!