You have to use a light adhesive masking tape. Also not as wide. They have some out there that are 1/4 to 1/8" in width.
What I suggest to make it easy for you would be one of two things.
Approach 1: With the thin masking tape, using two pieces, create the shape of the stripe.
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By creating the shape of the stripe, you will paint within the lines so to speak. (Here's where the coloring book practice comes in.
So stay in the lines.) With broad yet soft brush and using as minimal strokes as possible, paint the stripe within the lines provided by the tape. You want the finish to match the rest of the body and have that spray painted look.
Approach 2: This one is a bit more complicated and can mess up the model if not done correctly so be careful. Get some paper. Using the diagram of the car draw on the paper the shape of the stripe. Then, cut the paper so that the space between the lines(stripe) is cut out. You might want to cut the paper in two just to make it easier when you lay it over the car. Apply the masking tape so that 1/8" comes passed the edge of the paper and adheres to the car. Now you should have the body of the car covered with the paper and only the part that will be the stripe is exposed. You can now use spray paint to color in the stripe. Once done, let the paint dry and set. Do not remove the maskine tape until it is fully set. I know this sounds complicated, but if done right, will produce magnificent results every time.