I would make sure my computer could write to an SD CARD, then get an SD card shield, first. Now, please realize this post is to show one way of doing it; not to show YOU how to do it.
I would then code an Arduino routine to read the data from the card and parse it into an array of the coordinates.
Your code to move the steppers would then use this array.
That's why I asked if you have ever coded anything before. With the Arduino (or any microcontroller) there will be a bit of that.