The very first task to do on this project was to figure out the most important question:

How do you play a guitar with on hand?

Of course, there are two functions that your hands perform when playing a guitar; strumming and fretting. So which function do you replace? 

I saw plenty of youtube videos under "robotic guitars," and even the best fretting concepts were bulky and limiting. I decided to design the system to replace the strumming hand, since performing the strumming seemed easier than trying to figure out how to emulate the seemingly infinite number of fretting possibilities without compromises that would limit improvisation and flexibility of voicing and solos. 

So now that I knew I'd be trying to design a strumming device, the question became how to do the actuation. The first concept was to use a single arm with a pick at the end that would swing back and forth somehow to strum the strings, kinda like this:

It's a simple concept, but the thing I didn't like was that it didn't lend itself to selective picking, or picking of multiple strings simultaneously. Since this was a project for a maturing musician, I knew that she would quickly outgrow those limitations - I needed selective / simultaneous picking capability, and the single concept arm won't cut it. I need an independently actuated pick for each string.

Some more digging on the internet turned up a few projects that used individual hobby motors to pick the strings, like this one (which is more recent, but uses the same concepts in videos I saw when I was at this stage):

Next: how to control it? I actually started off thinking I would do computer control with a Wii-mote strapped to the foot or a foot pedal,  but then I came across Force Sensitive Resistors (FSRs). If you're like me, and didn't know what an FSR was, check out Sparkfun's tutorial here:

https://www.sparkfun.com/products/9674

I immediately came up with a concept controller, which consisted six FSRs adhered to a rigid base, and laid out in a way where the musician could kind of "strum" across the sensors with their foot like they would if the guitar were at their feet. I found FSRs and got to work. The first prototype only took me about 4 hours to make (once the FSRs arrived).

I milled a base out of acrylic with a recess for the wiring.

...then cut the FSRs to length, glued them in place, and wired them up:

I figured I'd want some way to feel the sensors with my foot, so I cut some balsa strips and adhered those with spray adhesive to the center of each sensor:

Lastly, I covered the whole thing in nice soft felt, and a quick layer of contact paper (remember covering books in school?) to give it a slippery surface for a stocking'd foot.  I also put an adhesive rubber pad on the back to prevent it from slipping on the floor, and a cover for the wire area. Here's the very first prototype of the foot controller!

The last thing to do was test, and unfortunately I don't have any more pictures of that process. But it worked exactly as it was supposed to! Now that I knew I had a control concept that worked, it was time to figure out how to strum the guitar.