Intalling grills/duct covers

Hi, i had an HVAC system retro-fitted in my home. The installer left some of the work undone..such as not installing a humidifier on one unit and failing to cover the duct openings (the part that run to the rooms).
Can someone point me to a site or documentation on how to install the grills on these openings? Are they screwed into the sheet rock or into the duct itself somehow? Is there anything special i need to be aware of when adding these grill covers?
Also, are register sizes standardized? Meaning, am i likely to find the grill covers at home depot that will fit the registers?
thanks much
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