External Trigger - Ganglion

in Ganglion
Hello all,
Best regards,
I am required for my master's thesis project to synchronize external visual stimuli with the EEG data acquisition from the Ganglion board.
The external visual stimuli will be detected by a phototransistor circuit. I have searched this forum and as I understand it, it is possible to read from one of the Analog Inputs of the board, and insert this value in the Aux column in the final EEG txt file produced.
This can only be done by re-programming the board's firmware. Currently the version on mine is 1.0.0
I know that it can be re-programmed over the air but I don't know where to start. I know that the libraries can be found here: https://github.com/OpenBCI/OpenBCI_Ganglion_Library/tree/master/OpenBCI_Ganglion_Library
But what files exactly do I edit/export to the Ganglion? I really just want to access one or two of these analog pins and add them to the Aux channels (I don't need the accelerometer values).
It would be a huge help if someone could give me some hints on this.
It would be a huge help if someone could give me some hints on this.
Best regards,
João Santos