How to do it ? . I don't know
Install Dannye's main.scm and then while driving a cabbie press shift or the general horn button. If you don't wanna use that, finish the taxi missions. Your cabbie can now jump with the buttons mentioned before. Then just try something easy and mimic what people did here. It takes time to learn but once learned it's not incredibly difficult do get some height constantly.
EDIT://Last 5 stunts made the video btw.