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First, the rough opening will need to be prepped for the new door. To protect against moisture, apply two layers of sill flashing tape to the bottom of the opening and partially up the sides.
The new door should be 3/4" more narrow than the rough opening and 1/2" shorter. Cut 1" tabs in the sill flashing tape ½" from each side of corners. This will enable the tape to lay flat as it wraps around the corner and up the sides of the rough opening. The tape should run about 6" up the sides of the frame.
Place three beads of sealant around the opening: one 3/4 inch from the exterior, one where the threshold will go, one 1/4 inch from the exterior edge of the wood blocking. After a few beads of sealant are laid down, set the bottom of the door in place, and then tilt the upper part into the opening. This way the frame won’t smear the sealant.
If you keep the plastic shipping spacers on the frame, it will help keep everything square. Use as many shims as necessary to make the frame plumb and square and then the spacers can be removed.
Insert insulating foam sealant between the doorframe and the rough opening. Three-inch screws can be driven through each hinge into the structural framing to hold the door in place. Once the door has been installed, seal around the outside perimeter with an exterior sealant.