If you decide to have the new porch light fixture on an existing circuit with capacity, run the wire to the nearest light fixture or outlet on that circuit. When the wire is run to its final ...

Stuff the wiring inside of the junction box. Most light fixtures come with trim to cover the hole in the wall or purely decorative trim. Follow the manufacturers to finish up the installation.

However, there is a stud in the EXACT center so if I mount a junction box to the side ... then run an electric wire through the stud or along-side the stud and into the light fixture.< ...