I got a roll of that stuff and gonna try to see what all the hype is about...
Ok here is a trick I saw the pro's do. Use a clothes steamer for the tinted vinly stuff you might want to put over your headlights to darken them. They used a lot of soapy water to fit into place then used the steamer to get it to mold to the angles and curves. I would not have thought about the steamer...it gives off water therefore making the vinyl easier to work with. Use a hair dryer also on the other vinyl stuff but I wonder if the steamer would work the same.
oh and use very sharp exacto blades e.g. the hobby kit that has the long pen like holder and you can swap different style blades into depending on what you need (angle cut, curves, etc...)