Remove the old register from the vent needing to be covered.
Measure the vent carefully. Most vents are a standard size; however, if the vent was not cut properly, a customized register may have to be ordered.
Purchase the new register. Many home stores sell these registers or they can be bought online. Custom made registers will be more expensive than a standard register.
Place the new register over the vent. The fit should be snug, but if it is a little tight or a little loose, it will probably not affect the airflow of the house.