To do a reverse 180 you need to be driving in reverse by holding down LT and then once in the zone push the left stick left or right until your car is 90 degrees and then let go of LT and hold down RT in order to accelerate and get your car round the 180 degrees.
A good scene to do this in is the third scene in Odd Jobs. After just 10 seconds in you will have a chance to do the stunt.
Tip 1: - Put Easy mode on to give you an extra strike
Tip 2: - When reversing in to the box give yourself an advantage by heading into it at a slight 20 degree angle or so.
Tip 3: - It doesn't need to be done in one fluid motion. Therefore if your finding it difficult then take it slow and make sure you dont exit the box