For help on cabling your GRBL board, please visit https://github.com/gnea/grbl/wiki/Connecting-Grbl.
Adjust stepper current
If you didn't adjust the current of your stepper drivers yet, do it now.
It doesn't make sense to configure acceleration and max feed when the stepper current is not correct! Set the current to a value where you have good torque but don't overheat the driver and motor.
See the documentation of your machine or stepper drivers on how to adjust the current. Details can be found on http://reprap.org/wiki/Pololu_stepper_driver_board.
You need to know how many stepps each axis has to go for 1mm distance. This is depending on pully sizes or spindle pitch and microstepping.
There is a nice calculator on http://www.prusaprinters.org/calculator/#steppermotors.