Procrastineering has an excellent article titled: Simulated PS2 Controller for Autonomously playing Guitar Hero – Ok, so it’s not quite a robot… but still, it’s pretty slick.

Here is a sample of the autonomous game:

But roughly, what I do for playing Guitar Hero is:

1. Unwarp the fret board so it is square (using a homography)
2. Use a template match to find notes along each column.
3. Track notes over time to ensure they are not spurrious.
4. Once notes hit a trigger line, queue the button press.