It's easy. I do them in Notepad, I draw them like this

.|||||.|||||.|||||.|||||||.
.|.....|...|.|...|.|..|..|.
.|.....|...|.|...|.|..|..|.
.|||||.|||||.|||||.|..|..|.
.....|.|.....|...|.|..|..|.
.....|.|.....|...|.|.....|.
.|||||.|.....|...|.|.....|.

(probably won't show)

Then I replace those (in the single replace feature of notepad) to this

: mrgreen::mrg reen::mrgree n::mr green:
:mr green:: mrgreen:
:mr green:: mrgreen:
: mrgreen::mrg reen::mrgree n:
: D:mrgreen :
: D
: mrgreen::mrgre en: