Prep the Area
Install a water resistive barrier at top and bottom of opening.
Attach sill-flashing tape to sill.
Attach a sill support to the exterior of the box plate, flush with the subfloor.
Remove screens, hardware and/or grilles from new door. Do not remove plastic shipping spacers yet; they will keep the door square during installation
Place three beads of sealant inside the entire 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.