How to spray paint parking lot stripes by hand and avoid overspray
Make a stencil. That what road markers typically use for arrows, symbols, letters and numbers .

Easiest would be out of some cardboard boxes. Flatten the boxes, cut them and create a stencil with a gap shaped like the stripe. Place on the road, spray paint, then re-position for the next stripe.

Illustration from Asphalt Line Stripping: