What kind of car is this? If you are planning stock "Rally stripes" on say, a late sixties Camaro or Chevelle, and I assume Mustangs and such, they all have stencil kits that make it a BREEZE to spray them.
I sprayed many of those stripes masking them off with a ruler and my eye. After the first time I used a stencil kit, I was hooked. Never will I go back to free hand.
Paint the car first and then using the stencil kit you are done in only an hour or so, it is that easy.