Child Theme help

I’m attempting to create a child theme - downloaded the Child Theme Configurator plugin.

Would like to create a second side bar and am attempting to follow the instructions provided in this post -

#5 in the above referenced post says:
Tweak Bootstrap layout from col-md-8 and col-md-4 to something like col-md-3, col-md-6, col-md-3

Where is the Bootstrap layout - or what is the file called?