How do you install a mounting plate for a ceiling light?

The mounting plate for a ceiling light is designed to be mounted to a round or octagonal ceiling box. Very often these can be installed in existing structures without too much trouble. Without the box it is a code violation. Splices must be in enclosures and hanging loose in the ceiling doesn't qualify.

If you do not have a box in the ceiling, you might consider installing a box ON the ceiling. It would be only 1 to 1 1/2 inches deep. Your splices would be safely inside. These boxes are typically used in outside installations and come in white or gray and take paint nicely so no one may ever notice. This type of installation is acceptable if the box is properly mounted.