Basically you have to score 5,000 points by doing a Manual trick.
Way easier than you might think with the help of Slo Mo
I did this on Goatville, and I found that using one of the sidewalks on the main road is a nice clear pathway to do this on.
So just activate Slo-Mo, then start a Manual by quickly pushing Up then Down or Down then Up on the Left Thumbstick
You have to end the Manual once you get to 5,000 though, it won't count if you crash.