You can buy tape-on pinstripes at auto parts stores. An auto accessory store may have a better brand, but the others will work ok.
Just want to re-emphasize what Salty said. After you tack the pinstripe on one end of the car for a starting point, then stretch it the full length of the car before cutting off and peeling. (don't try this on a windy day nor on a real hot day in direct sunlight)
When you get the tape stretched out full length, stoop down at eye level. You'll see just how straight you are. Before pressing any of it down, you can lift it back up to reposition it to get it where you want it. When you do that, don't pull up too hard so as not to pull up the tacked down end. Once you got it where you want it, break it off, but don't make any cuts yet.
Next press the tape down straight into the car at intervals first, then the whole length at once. Then and only then start peeliing the clear cover tape. Peel at the sharpest angle possible so that you don't pull up the pinstripe as you're doing that. After peeling, you can make your cuts for the doors and both ends. Most people bring the ends together to a point at least on the fronts, but it's not easy to do for a first timer, so you might pass on that.