Yet another way:
:'<,'>g/^/norm I//
/^/
is just a dummy pattern to match every line. norm
lets you run the normal-mode commands that follow. I//
says to enter insert-mode while jumping the cursor to the beginning of the line, then insert the following text (two slashes).
:g
is often handy for doing something complex on multiple lines, where you may want to jump between multiple modes, delete or add lines, move the cursor around, run a bunch of macros, etc. And you can tell it to operate only on lines that match a pattern.