*Please make sure that your motors are properly tuned before setting the limits or they won't work properly.
If you're using your Homing Switches, you can ignore that, and simply "Reference All Home" so that your "Machine Coordinates" Home is set automatically at Zero.
A red border should appear around the button (see picture below)
Make sure to use Zero as one dimension and the width/length as the other. Note that the Z axis is set opposite from the others. See picture.