The picture is self-explaining. The small disc contains the originals chords of the song, and great the wished ones.
Both discs are united by the center and turn one on the other.
I used a pair of washers to fill up the central hollow of discs, and another pair of greater washers to unite the set with an aluminum rivet. A screw can also be used, or what the talent suggests to you.
In the case of the photo (the first photo), a song in E will be transposed to A.
D --> G
C --> F
F --> A#
... and so
(Pardon my deplorable english. I speak spanish.)