In a past life I have striped a dozen dozen cars. And more recently a few boats. Boats are more difficult for several reasons of which one you mentioned.
Not sure I can help much as I could do it much easier than I can describe it.
Most likely you are securing one end of the tape, then stretching out a few feet of tape and holding it close but above the surface as you "sight" down the tape. Then slowly moving the tape closer as it begins to lightly touch the surface beginning at the secured end and moving towards your position. Never actually touching the tape to press it down until the end.
Then before it contacts all the way to your position, stretching out more feet and repeating. Never working right on top of the contact point is important. As is not pressing it down as you go. It can be lifted and adjusted this way. And press down firmly at the end.
Here's an idea for a curve. The long tail of tape is a tangent of the curve being created. The tail is a tangent from the point of conact on the surface. From a distance this can be visualized. For a mostly horizontal surface as you slowly move the tape closer your hand motion slowly moves up or down. Slow and smooth, eyes on the contact point.
Also, I often place small marks of tape or pencil along the curved path above or below as appropriate, maybe a foot or two apart. Then my eye shifts back and forth from the contact point to my target marks. As I move the tape slowly and smoothly, very important. Sometimes I get it on the first try, sometimes not. Not pressing as I go makes it easy to lift
and adjust. But it is important to never pull hard enough to stretch the tape if it's vinyl.
"placing stripes on car or boat youtube" for help maybe.
My data is throttled for seven more days so can't watch vids to offer good links.