the quick & easy way is cutting on the car using masking tape or fineline as a guide. I would do it this way if for a car yard or really in a hurry. Just make sure you use a new blade & keep it low. I try to use the extended blade as a visual guide along the masking tape & make one continuous cut. The main problem is cutting a smooth line the whole way. Stripes on bonnets (hoods for you americans) are too close to the eyeline & show up tiny wobbles where you've moved further down the panel or bumped your elbow on the mirror or something.
If it was my car & that bodyline is as straight as it looks, I'd put down 2 lines of masking tape at the edges then lay over some butchers or tracing paper & run a pen down the masking tape to get a stencil. Lay that stencil on the vinyl & use a straight edge to cut. Fit vinyl to car between masking tape. Nice straight lines unless you cant cut using a straight edge, & no cut paint either