This is a somewhat simple way to calibrate your A4988 stepper driver board. keep in mind it's not perfect, but will allow you to get very close to a good configuration.

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A 4988 Datasheet:
https://www.pololu.com/file/0J450/A49...

Pronterface download:
http://www.pronterface.com/

Tools:
Multimeter
alligator clip
Trim pod Screwdriver or flat head screw driver with non conductive handle
MKS GEN L v1.0/RAMPS 1.4 motherboard
A4988 stepper driver board with heat sink
USB serial cable
a computer for USB power