Firstly, measure the lengths of the tracks you'll need for the top and bottom of the door and transfer these measurements to your tracks with the pencil.
Now that you've marked up the tracks with your measurements, put on your safety gear and cut the tracks to size using your angle grinder.
Use your spirit level to make sure the floor is level before laying the tracks on the floor. Pack up one end if it isn't. Make a pilot hole in your floor through the track's pre-drilled hole. Then screw the tracks down in place with your drill. You'll need someone else to help you hold the track when screwing in the top one.
Place the rear door into the opening by putting it into the top track first. Then carefully lower the door into the bottom track. Next put the second door into place, starting from the top. Finally, test the doors to see if they close securely.