I can tell you this much. I decided to do without it on my tank after spending a bunch of hours masking and still not liking the results! I might have done better free hand for all I know, but getting the bends just right and getting them to match was beyond my patience level. And skill apparently! My hat is off to those who have mastered it and I would be grateful for any tricks used.
Even the guy who does it free hand in the video is following a chalk line or something. I have seen similar things done where you take an overlay that has pin holes in it (made by a pizza cutter looking thing with points on it) and use a chalk bag to slap it a few times. When you remove the overlay, you have a series of dots to connect. When I was fresh out of high school I worked in a place that had a sign shop. It was kind of interesting that I was next door building circus tents that got rented out for fairs and exhibitions. The sign shop had three guys in it that would hand paint custom signs for the exhibitors. One of these guys hand striped a '34 Chevy roadster. He used plastic like one would use for a boat curtain to make his overlay. Once he figured out how to get it to lay on the fenders he was able to duplicate it in reverse for the opposite side. Mostly the chalk line created was to keep perspective of size and not as a guide to make a straight line. Even on hand painted signs the guys would lay out a faint grid in chalk so that there lettering could be kept uniform. But the point of the story I guess, is that he spent 40 hours a week doing this kind of work and was really good with the hand-eye coordination required. My experience with this type of thing is that confidence is the main ingredient. Sadly in our digital age, there are not a lot of hand painted signs and it is an art that doesn't get a whole lot of practice. Back then it was acceptable to have things be a little less than perfect because it was hand done. I don't mean sloppy, but obviously done by a human. Now we have been trained that this is not acceptable. If you don't believe me, just read the threads here about parts that need fettling before use!
Russ