round ceiling registers installation

jp1953

i need to add 3 8' boots and 3 6' boots the squar boot has a flange to holed boot in place and when you attach the grill it will keep it in place the round bbot is installed in the end of the duct pipe so my question is do i let the grill hole in place the same as the squar boot , but the round boot has no flange to holed it in place so do i make my hole in ceiling 8" and let screws go through the grill and sheet rock and then into the boot to hole it in place

coolmen

the register screw must go through the sheet rock and screw into the metal boot it self. 2" white head screws work great