You have a couple options, and I'm sure somebody from 254 will stop by shortly to explain their method, but here's what I like to do:
Draw a perpendicular line segment to each point on your original trajectory. The center of the segment should lay on the trajectory and its length should be the width of your robot.
Alternatively, don't generate separate left/right tracks and instead include heading as a part of your trajectory plan. Have your feedback loop include that heading to modify the left/right control efforts
Edit to provide some resources:
https://www.chiefdelphi.com/forums/s...d.php?t=126639 (A discussion of effectively how to create those paths)
https://www.chiefdelphi.com/forums/s...d.php?t=130769 (Some planners that inherently create individual left/right paths)