Before doing fancy stuff with the steppers they need to be calibrated. This is an essential step and must not be left out.
I have found a nice and explanatory video tutorial over at BuildYourOwnCNC.
In generall it says you need to calculate the estimated step/mm.
From that point you move your desired stepper via gcode (x200 for example). Then you need to take the discrepancy and calculate your new step/mm until it moves the exact range you commanded. But see the video for more information ans some math.
I suggest to create an excel sheet to save you some headache.
You can use every kind of terminal tool to communicate with grbl. I used CoolTerm.
I guess you know how to load a terminal and connect to your Arduino.
In the picture you can see my current calibrated data.