This kind of sensors measure muscle activity by detecting its electric potential.
You will find below two example of such sensors : - https://www.sparkfun.com/products/13723 - https://www.seeedstudio.com/Grove-EMG-Detector-p-1737.html
You will have to be careful when positioning the electrodes on the muscle: two of them have to be placed on the muscle (parallel to the muscle fibers) for which you want to measure the activity. The third electrode serves as a reference, and has to be placed outside the muscle area.
You will find plenty of links and instructions on the web on how to use those sensors.
You will have to use the 3 pins "+", "-", "sig" (as shown below on to the right of the sensor).
You will have to connect the sensor to a 3.5mm stereo male jack, using 3 (approx. 1 m long) cables - one for Vcc, one for the Ground, and one for the Signal.
You can also use one or two sliders to play with the Baah Box. always using the same connection as the myoware sensor. (Actually, any analog arduino sensor will do the trick)
You can use an Arcade Joystick with its connection cable, like this one.
And cut one end of the cable to solder it to a male DB9 connector.
The table below shows the connection to the DB9 male connector to the joystick pins.
You will then be able to connect the Baah Box and the joystck with a DB9 male-female cable - and reuse the joystick for another use!
The final step is to put the joystick (and the DB9 connector soldered to it) in a home-made box, like so :
