No heat gun, just water and a plastic squeegee. Make sure the surface is clean. Cut the Scotchguard to shape. Spray the adhering side and the surface with water. Position it in desired location, then squeegee the water out, which will adhere it to the surface. Make sure no air bubble get trapped and that the edges adhere. Its a little tricky due to the curvature of the headlights, but it only took us about 15 minutes total.
Of course, it may take you or eye a bit longer, he's a bit of a pro at this point, having taped 90% of his new Camaro. lol
The only thing dependable about the future is uncertainty.