This will explains the basic process of adding an External Stepper to the MKS Gen L 1.0 board. It also explains how to add an addition stepper.

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Required Hardware:
MKS Gen L 1.0 board and USB cable
Stepper Motor Driver Control Extension Shield Board For A4988 DRV8825

Required Software:
Arduino studios 1.8.5 or higher
https://www.arduino.cc/
Pronterface.exe
http://www.pronterface.com/
MarlinFW (Firmware v1.1.8 or higher)
http://marlinfw.org/meta/download/