Slide the door frame into the doors' rough opening. Center the frame in the opening.
Hold a bubble level on one of the door frame's vertical sides and adjust the vertical side until it rests plumb, using the bubble level as a guide. Slip shims between the door frame's plumb side and the building's rough opening.
Tack the plumb side of the French doors' frame to the building's rough opening with 3-inch drywall screws, using a drill to tighten the screws. Place a screw 8 inches from the top and bottom of the frame's side.
Tack the French door frame to the building's rough opening with the 3-inch drywall screws. Place a screw 8 inches from each side of the frame and in the middle.
Hold the bubble level against the top of the doors' frame. Adjust the top of the door frame until it rests level in the rough opening, using the drywall screws to raise or lower the frame as needed. Tightening the screws raises the frame and loosening the screws lowers the frame.
Hold the bubble level against the remaining side of the frame. Adjust the side of the frame for plumb, using the bubble level for reference. Slip shims between the door frame's side and the rough opening. Hold the remaining side of the frame in still with 3-inch drywall screws, placing a screw 8 inches from the top and bottom of the frame and in the middle.
Install the French doors on the door frame. Close the doors and check the gaps on all sides of both doors. If one door rests lower than the other, remove the tack screws on the low door's vertical frame and raise the horizontal part of the frame above the door with the tack screws. Reinstall the tack screws on the vertical part of the frame and recheck the doors against each other.