Using bench, install ERPNext as mentioned here.
Once ERPNext is installed, add MPesa B2c app to your bench by running
$ bench get-app https://github.com/navariltd/navari-mpesa-b2c.git
After that, you can install MPesa B2C app on required site by running
$ bench --site [site.name] install-app navari_mpesa_b2c
MIT. See license.txt for more information.