Confession time – I've been absolutely terrible at this since I started modeling 30+ years ago. I recently finished the new Hasegawa Stuka and lo and behold, a spinner decal.
I fought with it for nearly two hours with no luck, I couldn't get it to settle down and line up, much less conform to the curves of the spinner.
I tried parafilm, not much better. I finally decided to try my hand at hand painting, which is relatively easy on smaller scales but pretty noticable on 1/32 scale.
I found a piece of brass rod that fit the spinner, chucked it in my lathe and adjusted the speed of the lathe as low as it could go. Then, with a liner brush and a steady hand, painted a line from the tip of the cone to the base. It was perfect, only had a couple of small touch-ups to make.
If you don't have a lathe, try a variable speed drill with a lock setting. The lower you can adjust the speed, the easier it is to paint the stripe.
Jeff