Install software and connect the device, watch and follow the instructions of the video under "learn & support": https://www.tobiipro.com/product-listing/tobii-pro-x3-120/
At this point you should have installed "Installation and Configuration Tool for Tobii Pro X2 and X3 eye trackers", which can be found below the video that is linked above.
Install the eyetracker manager. Use the manager to calibrate the eyetracker to get correct readings.
Follow instructions on this site, and place the sdk in backend/crunch/eyetracker